Fix musl build

This patch fixes cross-compiling U-Boot tools with the musl C library:
  * including <sys/types.h> is needed for ulong
  * defining _GNU_SOURCE is needed for loff_t

Tested for target at91sam9261ek_dataflash_cs3.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Tom Rini <trini@konsulko.com>