grub2: fix build on ArchLinux

add dependencies on libfreetype,libbz2,libpng
for compilation of grub-mkfont

build error was:
...i486-openwrt-linux-uclibc/bin/ld: cannot find -lfreetype
...i486-openwrt-linux-uclibc/bin/ld: cannot find -lbz2
...i486-openwrt-linux-uclibc/bin/ld: cannot find -lpng16
collect2: ld returned 1 exit status
Makefile:16782: recipe for target 'grub-mkfont' failed

Signed-off-by: Dirk Neukirchen <dirkneukirchen@web.de>