libmnl: fix build when bash is not located at /bin/bash

This fixes the libmnl build on macOS, which ships with an outdated bash
at /bin/bash. During the OpenWrt build, a modern host bash is built and
made available at staging_dir/host/bin/bash, which is present before
/bin/bash in the build's PATH.

This is similar to 8f7ce3aa6dda, presently appearing at
package/kernel/mac80211/patches/build/001-fix_build.patch.

Signed-off-by: Mark Mentovai <mark@mentovai.com>
(cherry picked from commit beeb49740bb4f68aadf92095984a2d1f9a488956)

package/libs/libmnl/patches/001-fix_build.patch

@@ -0,0 +1,11 @@
+--- a/doxygen/Makefile.am
++++ b/doxygen/Makefile.am
+@@ -21,7 +21,7 @@ doxyfile.stamp: $(doc_srcs) Makefile.am
+ # The command has to be a single line so the functions work
+ # and so `make` gives all lines to `bash -c`
+ # (hence ";\" at the end of every line but the last).
+-	/bin/bash -p -c 'declare -A renamed_page;\
++	bash -p -c 'declare -A renamed_page;\
+ main(){ set -e; cd man/man3; rm -f _*;\
+   count_real_pages;\
+   rename_real_pages;\