[build] Fix building elf2efi using binutils 2.20

When using binutils 2.20, it seems to be necessary to add -ldl to link
against -lbfd.

Reported-by: Duane Voth <duanev@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

 ELF2EFI_CFLAGS	:= -I$(BINUTILS_DIR)/include -I$(BFD_DIR)/include \
 		   -I$(ZLIB_DIR)/include -idirafter include \
 		   -L$(BINUTILS_DIR)/lib -L$(BFD_DIR)/lib -L$(ZLIB_DIR)/lib \
-		   -lbfd -liberty -lz -Wl,--no-warn-search-mismatch
+		   -lbfd -ldl -liberty -lz -Wl,--no-warn-search-mismatch
 
 $(ELF2EFI32) : util/elf2efi.c $(MAKEDEPS)
 	$(QM)$(ECHO) "  [HOSTCC] $@"