gtest: fix poisoned exit in compile flags

visibility.h is included globally in PX4 via cmake compile flags.
It contains poisoning the exit() command which is used by gtest
to close the test application. Removing the flag for gtest compilation
fixes the compile error:
gtest.cc:4757:7: error: attempt to use poisoned "exit"

src/include/visibility.h

@@ -86,7 +86,7 @@
 #ifdef __cplusplus
 #include <cstdlib>
 #endif
-//#pragma GCC poison exit
+#pragma GCC poison exit
 
 #include <stdlib.h>