Define ST_COMPILATION in the St enumeration template

In parallel builds we may end up with st-enum-types.c being built inside
separate targets outside of src/st which may not have the ST_COMPILATION
pre-processor symbol defined. For this reason, we need to define it
ourselves in the source file, before including other headers, to avoid
the single-include guard.

src/st/st-enum-types.c.in

@@ -1,4 +1,7 @@
 /*** BEGIN file-header ***/
+#ifndef ST_COMPILATION
+#define ST_COMPILATION
+#endif
 #include "st-enum-types.h"
 /*** END file-header ***/