[build] Handle R_X86_64_PLT32 from binutils 2.31

Starting from binutils 2.31.0 (commit bd7ab16b) x86-64 assembler
generates R_X86_64_PLT32 instead of R_X86_64_PC32.

Acked-by: John Jolly <jjolly@suse.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/util/elf2efi.c

 		case ELF_MREL ( EM_ARM, R_ARM_THM_JUMP24 ) :
 		case ELF_MREL ( EM_ARM, R_ARM_V4BX ):
 		case ELF_MREL ( EM_X86_64, R_X86_64_PC32 ) :
+		case ELF_MREL ( EM_X86_64, R_X86_64_PLT32 ) :
 		case ELF_MREL ( EM_AARCH64, R_AARCH64_CALL26 ) :
 		case ELF_MREL ( EM_AARCH64, R_AARCH64_JUMP26 ) :
 		case ELF_MREL ( EM_AARCH64, R_AARCH64_ADR_PREL_LO21 ) :