Fix missing return statements that are seen with -Werror=return-type.

Error example:

Code:

        #include <locale.h>
        int main () {
            /* If it's not defined as a macro, try to use as a symbol */
            #ifndef LC_MESSAGES
                LC_MESSAGES;
            #endif
        }
Compiler stdout:

Compiler stderr:
 In file included from /usr/include/locale.h:25,
                 from /tmp/tmpep_i4iwg/testfile.c:2:
/usr/include/features.h:382:4: warning: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Wcpp]
  382 | #  warning _FORTIFY_SOURCE requires compiling with optimization (-O)
      |    ^~~~~~~
/tmp/tmpep_i4iwg/testfile.c: In function 'main':
/tmp/tmpep_i4iwg/testfile.c:8:9: error: control reaches end of non-void function [-Werror=return-type]
    8 |         }
      |         ^
cc1: some warnings being treated as errors

1 files changed, 1 insertions, 0 deletions

diff --git a/mesonbuild/compilers/c.py b/mesonbuild/compilers/c.py
index 47cfd98..eff7161 100644
--- a/mesonbuild/compilers/c.py
+++ b/mesonbuild/compilers/c.py
@@ -67,6 +67,7 @@ class CCompiler(CLikeCompiler, Compiler):
             #ifndef {symbol}
                 {symbol};
             #endif
+            return 0;
         }}'''
         return self.compiles(t.format(**fargs), env, extra_args=extra_args,
                              dependencies=dependencies)