Add a trailing comma to enum

Should help produce better diffs and clang-format won't squash the enum
on one line.

GIT_SILENT

src/imageformats/gimp_p.h

@@ -41,7 +41,7 @@ const uchar OPAQUE_OPACITY = 255; //!< Opaque value for 8-bit alpha component.
 typedef enum {
     RGB,
     GRAY,
-    INDEXED
+    INDEXED,
 } GimpImageBaseType;