[efi] Include SNP NIC driver within the all-drivers target

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

src/arch/x86/Makefile.efi

 NON_AUTO_MEDIA	+= drv.efi
 NON_AUTO_MEDIA	+= efirom
 
+# Include SNP driver in the all-drivers build
+#
+DRIVERS += snp
+
 # Rules for building EFI files
 #
 $(BIN)/%.efi : $(BIN)/%.efi.tmp $(ELF2EFI)