Fix inverted condition in _Atomic declaration

fixes #1593

common.h

@@ -642,6 +642,7 @@ void gotoblas_profile_init(void);
 void gotoblas_profile_quit(void);
 
 #ifdef USE_OPENMP
+
 #ifndef C_MSVC
 int omp_in_parallel(void);
 int omp_get_num_procs(void);
@@ -649,12 +650,15 @@ int omp_get_num_procs(void);
 __declspec(dllimport) int __cdecl omp_in_parallel(void);
 __declspec(dllimport) int __cdecl omp_get_num_procs(void);
 #endif
+
 #if (__STDC_VERSION__ >= 201112L)
+#include <stdatomic.h>
+#else
 #ifndef _Atomic
 #define _Atomic volatile
 #endif
-#include <stdatomic.h>
 #endif
+
 #else
 #ifdef __ELF__
 int omp_in_parallel  (void) __attribute__ ((weak));