some warnings to fix

[illwieckz]

some warnings to fix:

In file included from /home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:28:0:
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/navgen.h:101:1: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 const int              getNumVerts() { return nverts; }
 ^~~~~
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp: In function ‘void LoadBrushTris(std::vector<float>&, std::vector<int>&)’:
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:178:55: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
  ApplySurfaceParm( "default", &solidFlags, NULL, NULL );
                                                       ^
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:179:52: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
  ApplySurfaceParm( "playerclip", &temp, NULL, NULL );
                                                    ^
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:183:53: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
   ApplySurfaceParm( "sky", NULL, &surfaceSkip, NULL );
                                                     ^
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp: In function ‘void LoadPatchTris(std::vector<float>&, std::vector<int>&)’:
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:551:55: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
  ApplySurfaceParm( "default", &solidFlags, NULL, NULL );
                                                       ^
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:552:52: warning: ISO C++ forbids converting a string constant to ‘char*’ [-Wwrite-strings]
  ApplySurfaceParm( "playerclip", &temp, NULL, NULL );
                                                    ^
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp: In function ‘void rcFilterGaps(rcContext*, int, int, int, rcHeightfield&)’:
/home/illwieckz/dev/netradiant/tools/quake3/q3map2/nav.cpp:980:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for ( int i = 0; i < spanData.size(); i += 3 ) {
                   ~~^~~~~~~~~~~~~~~~~