package/mmc-utils: fix build with SSP

package/mmc-utils: fix build with SSP

Set AM_CFLAGS to an empty value to avoid the following redefinition
error when building with our custom _FORTIFY_SOURCE:

/accts/mlweber1/rc-buildroot-test/scripts/instance-1/output/host/bin/mips-linux-gnu-gcc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Wall -Werror -Wuninitialized -Wundef -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os  -D_FORTIFY_SOURCE=1 -Wp,-MMD,3rdparty/hmac_sha/.hmac_sha2.o.d,-MT,3rdparty/hmac_sha/hmac_sha2.o -c 3rdparty/hmac_sha/hmac_sha2.c -o 3rdparty/hmac_sha/hmac_sha2.o
<command-line>:0:0: error: "_FORTIFY_SOURCE" redefined [-Werror]

Fixes:
 - http://autobuild.buildroot.org/results/cfef9315441b5f4909b58a6dccd8bea8e67ae992

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>