mmc-utils: don't set -D_FORTIFY_SOURCE in Makefile

mmc-utils: don't set -D_FORTIFY_SOURCE in Makefile

Now that security features are set globally, having the
FORTIFY_SOURCE option set in Makefile breaks the build when
CONFIG_PKG_FORTIFY_SOURCE_{1,2} is enabled as well.

arm-openwrt-linux-uclibcgnueabi-gcc  -Wall -Werror -Wuninitialized -Wundef -D_FILE_OFFSET_BITS=64 -D_FORTIFY_SOURCE=2 -Os -pipe -march=armv6k -mtune=mpcore -fno-caller-saves -fhonour-copts -Wno-error=unused-but-set-variable -mfloat-abi=soft -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro -Wp,-MMD,./.mmc.o.d,-MT,mmc.o -c mmc.c -o mmc.o
<command-line>:0:0: error: "_FORTIFY_SOURCE" redefined [-Werror]
<command-line>:0:0: note: this is the location of the previous definition
cc1: all warnings being treated as errors
Makefile:35: recipe for target 'mmc.o' failed

Fix this by removing -D_FORTIFY_SOURCE=2 from Makefile.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Michael Heimpold <mhei@heimpold.de>