Try to unbreak sanitizer-x86_64-linux-autoconf after r230318

The buildbot failed to build with

    error: variable ‘enable_fp’ set but not used [-Werror=unused-but-set-variable]

let's add a `(void)enable_fp;`.

llvm-svn: 230323

compiler-rt/lib/sanitizer_common/sanitizer_internal_defs.h

@@ -295,6 +295,7 @@ extern "C" void* _ReturnAddress(void);
   do {                                                             \
     volatile uptr enable_fp;                                       \
     enable_fp = GET_CURRENT_FRAME();                               \
+    (void)enable_fp;                                               \
   } while (0)
 
 #endif  // SANITIZER_DEFS_H