cmake: add OS2 to OPT_DEF_LIBC enable list.

CMakeLists.txt

@@ -242,7 +242,7 @@ if(USE_GCC OR USE_CLANG OR USE_INTELCC OR USE_QCC)
 endif()
 
 # Default option knobs
-if(UNIX OR MINGW OR MSYS OR (USE_CLANG AND NOT WINDOWS) OR VITA OR PSP OR PS2 OR N3DS OR SDL_CPU_ARM64EC)
+if(UNIX OR MINGW OR MSYS OR OS2 OR (USE_CLANG AND NOT WINDOWS) OR VITA OR PSP OR PS2 OR N3DS OR SDL_CPU_ARM64EC)
   set(OPT_DEF_LIBC ON)
 endif()
 if(WINDOWS OR MACOS OR IOS OR TVOS OR VISIONOS OR WATCHOS)