fix: use [dev-dependencies.compat] for gtest

gtest is a compat (non-modular) package in mcpp-index. Declare it
under [dev-dependencies.compat] so mcpp resolves the correct namespace.

Required for mcpp 0.0.10+ namespace-aware dependency pipeline.

Author: Sunrisepeak

mcpp.toml

@@ -10,5 +10,5 @@ repo        = "https://github.com/mcpplibs/cmdline"
 [targets.cmdline]
 kind = "lib"
 
-[dev-dependencies]
+[dev-dependencies.compat]
 gtest = "1.15.2"