Add missing headers to gif BUILD definition.

% :.../gif/giflib-5.1.4/lib$ ls
dgif_lib.c  gif_err.c	gif_hash.h	   Makefile.am	 openbsd-reallocarray.c
egif_lib.c  gif_font.c	gif_lib.h	   Makefile.in	 quantize.c
gifalloc.c  gif_hash.c	gif_lib_private.h  Makefile.unx
Change: 131121931

gif.BUILD

@@ -9,12 +9,18 @@ SOURCES = [
     "quantize.c",
 ]
 
+HEADERS = [
+    "gif_hash.h",
+    "gif_lib.h",
+    "gif_lib_private.h",
+]
+
 prefix_dir = "giflib-5.1.4/lib"
 
 cc_library(
     name = "gif",
     srcs = [prefix_dir + "/" + source for source in SOURCES],
-    hdrs = [prefix_dir + "/gif_lib.h"],
+    hdrs = [prefix_dir + "/" + hdrs for hdrs in HEADERS],
     includes = [prefix_dir],
     defines = [
         "HAVE_CONFIG_H",