[build] Be explicit about -fcommon compiler directive

gcc10 switched default behavior from -fcommon to -fno-common.  Since
"__shared" relies on the legacy behavior, explicitly specify it.

Signed-off-by: Bruce Rogers <brogers@suse.com>
Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile.housekeeping

@@ -418,6 +418,7 @@ CFLAGS		+= -Os
 CFLAGS		+= -g
 ifeq ($(CCTYPE),gcc)
 CFLAGS		+= -ffreestanding
+CFLAGS		+= -fcommon
 CFLAGS		+= -Wall -W -Wformat-nonliteral
 HOST_CFLAGS	+= -Wall -W -Wformat-nonliteral
 endif