util/spkmodem_recv: Add strict CFLAGS

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/spkmodem_recv/Makefile

 # SPDX-License-Identifier: GPL-2.0-or-later
 PREFIX  ?= /usr/local
 INSTALL ?= install
+CFLAGS ?= -Os -Wall -Wextra -Werror -pedantic
 
 spkmodem-recv:
-	$(CC) -o $@ $@.c
+	$(CC) $(CFLAGS) -o $@ $@.c
 install: spkmodem-recv
 	$(INSTALL) -d $(DESTDIR)$(PREFIX)/bin/
 	$(INSTALL) $< -t $(DESTDIR)$(PREFIX)/bin/