util/spkmodem_recv: Define CC in the Makefile

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

util/spkmodem_recv/Makefile

 # SPDX-License-Identifier: GPL-2.0-or-later
+CC ?= cc
 PREFIX  ?= /usr/local
 INSTALL ?= install
 CFLAGS ?= -Os -Wall -Wextra -Werror -pedantic