Include geanyplugin.h from geanyfunctions.h for compatibility

geanyfunctions.h used to bring all function declarations, and some
plugins depend on this side effect instead of properly including
geanyplugin.h directly.  So, reintroduce the behavior for
compatibility with those plugins.

plugins/geanyfunctions.h

@@ -23,5 +23,6 @@
 #ifndef GEANY_FUNCTIONS_H
 #define GEANY_FUNCTIONS_H 1
 
+#include "geanyplugin.h"
 
 #endif /* GEANY_FUNCTIONS */