diff: add round-1 gnulib objects to libdiff.a

Add the gnulib modules needed to resolve first-round link errors for
cmp/diff/diff3/sdiff: xmalloc, xalloc-die, xstrtoimax, quotearg,
diagnose, version-etc, version-etc-fsf, progname, c-stack, xstdopen,
propername-lite, cmpbuf, exitfail.

https://claude.ai/code/session_01WGAwvvTwDg2yknFkmZ3qzs

Author: claude

sys/src/ape/cmd/diff/mkfile

@@ -11,7 +11,18 @@ TARG=\
 LIB= libdiff.a$O
 
 LIBO=\
-	sigsegv.$O stdbit.$O
+	sigsegv.$O stdbit.$O\
+	xmalloc.$O xalloc-die.$O\
+	xstrtoimax.$O\
+	quotearg.$O\
+	diagnose.$O\
+	version-etc.$O version-etc-fsf.$O\
+	progname.$O\
+	c-stack.$O\
+	xstdopen.$O\
+	propername-lite.$O\
+	cmpbuf.$O\
+	exitfail.$O
 
 CMPO=\
 	cmp.$O system.$O