- Fix extern declaration according to definition

Author: felipensp

ext/gd/php_gd.h

@@ -61,7 +61,7 @@
 
 PHPAPI extern const char php_sig_gif[3];
 PHPAPI extern const char php_sig_jpg[3];
-PHPAPI extern const char php_sig_png[3];
+PHPAPI extern const char php_sig_png[8];
 
 extern zend_module_entry gd_module_entry;
 #define phpext_gd_ptr &gd_module_entry