Fix _PYRMQ_INLINE to adapt to changes in python3.7+ C API

Author: voltterra

Modules/_librabbitmq/connection.h

@@ -49,7 +49,7 @@
 # elif defined(__GNUC__) && !defined(__GNUC_STDC_INLINE__)
 #  define _PYRMQ_INLINE extern __inline
 # else
-#  define _PYRMQ_INLINE __inline
+#  define _PYRMQ_INLINE extern __inline
 # endif
 #endif