package/efivar: disable -Werror

package/efivar: disable -Werror

Fix the following build failure with gcc 10:

/home/buildroot/autobuild/run/instance-1/output-1/host/bin/aarch64-none-linux-gnu-gcc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2   -I/home/buildroot/autobuild/run/instance-1/output-1/build/efivar-37/src/include/   -specs=/home/buildroot/autobuild/run/instance-1/output-1/build/efivar-37/gcc.specs  -L.  -fPIC -Wl,-z,muldefs     -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -o efivar efivar.c -lefivar -ldl
In file included from efivar.h:28,
                 from efivar.c:40:
In function 'text_to_guid',
    inlined from 'parse_name.constprop' at efivar.c:157:8:
guid.h:106:2: error: 'strncpy' output may be truncated copying 8 bytes from a string of length 38 [-Werror=stringop-truncation]
  106 |  strncpy(eightbytes, text, 8);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Fixes:
 - http://autobuild.buildroot.org/results/fcba72d359f4128515560e9105384cd4deff5043

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