[build] Fix building on older versions of gcc

Older versions of gcc (observed with gcc 4.8.5 on CentOS 7) complain
about having the label "err_ioremap" at the end of a compound
statement in bios_mp_start_all().  The label is correctly placed,
since it immediately follows the iounmap() that would be required to
undo a successful ioremap() in the non-error case.

Fix by adding an explicit "return" immediately after the label.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/interface/pcbios/bios_mp.c

@@ -165,6 +165,7 @@ static void bios_mp_start_all ( mp_func_t func, void *opaque ) {
 	/* No way to handle errors: caller must check that
 	 * multiprocessor function executed as expected.
 	 */
+	return;
 }
 
 PROVIDE_MPAPI_INLINE ( pcbios, mp_address );