gnu: gd: Fix long patch filename.

This fixes the generation of a tarball with "make dist":

  tar: guix-1.3.0.10365-4c4f9/gnu/packages/patches/gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch:
  file name is too long (max 99); not dumped

* gnu/packages/patches/gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch:
  Rename patch to ...
* gnu/packages/patches/gd-Revert-fix-303-gdlib.pc.patch: ... this.
* gnu/packages/gd.scm (gd)[source]: Update patch filename.
* gnu/local.mk (dist_patch_DATA): Update patch filename.

gnu/local.mk

@@ -1123,7 +1123,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/gcc-9-strmov-store-file-names.patch	\
   %D%/packages/patches/gd-fix-tests-on-i686.patch		\
   %D%/packages/patches/gd-brect-bounds.patch			\
-  %D%/packages/patches/gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch	\
+  %D%/packages/patches/gd-Revert-fix-303-gdlib.pc.patch		\
   %D%/packages/patches/gdm-default-session.patch		\
   %D%/packages/patches/gdm-elogind-support.patch		\
   %D%/packages/patches/gdm-remove-hardcoded-xwayland-path.patch	\

gnu/packages/gd.scm

@@ -57,7 +57,7 @@ (define-public gd
                               ;; Drop when
                               ;; https://github.com/libgd/libgd/issues/691
                               ;; is solved.
-                              "gd-Revert-fix-303-gdlib.pc-use-Requires-instead-of-Libs.patch"))))
+                              "gd-Revert-fix-303-gdlib.pc.patch"))))
     (build-system gnu-build-system)
     (arguments
       ;; As recommended by github.com/libgd/libgd/issues/278 to fix rounding