The x86_64 EDK2 headers include a #pragma to mark all subsequent
symbol declarations and references as hidden if position-independent
code is being generated.  Since libgen.h is currently included only
after the EDK2 headers, this results in __xpg_basename() being
erroneously marked as having hidden visibility (if the compiler
defaults to building position-independent code); this eventually
results in a failure to link the elf2efi binary.

Fix by including libgen.h prior to including the EDK2 headers.

Originally-fixed-by: Doug Goldstein <cardoe@cardoe.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>