Fix spawn with -DSPAWN_TEST not to require GEANY_API_SYMBOL

As stated in spawn.c, "This module does not depend on Geany when
compiled for testing (-DSPAWN_TEST)."

src/spawn.c

@@ -59,6 +59,7 @@
 
 #ifdef SPAWN_TEST
 # define _
+# define GEANY_API_SYMBOL
 #else
 # include "support.h"
 #endif