[efi] Fix building elf2efi.c when -fpic is enabled by default

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>

src/util/elf2efi.c

 #include <sys/mman.h>
 #include <fcntl.h>
 #include <elf.h>
+#include <libgen.h>
 #include <ipxe/efi/Uefi.h>
 #include <ipxe/efi/IndustryStandard/PeImage.h>
-#include <libgen.h>
 
 #define eprintf(...) fprintf ( stderr, __VA_ARGS__ )