Fix compilation warning due to missing function declaration.

diff --git a/filters/php_bumpmap.c b/filters/php_bumpmap.c
index 6910f82..00a1bc7 100644
--- a/filters/php_bumpmap.c
+++ b/filters/php_bumpmap.c
@@ -121,6 +121,9 @@ php_filter_deinit()
 	return;
 }

+int
+_php_bumpmap_getfilterindex(char*);
+
 int
 php_filter_exec(Imlib_Image im, char *filter, HashTable *params)
 {