Without this building bareboxenv-target with gcc 7.3 fails with:

  CC      scripts/bareboxenv-target
/tmp/cc9G2oOY.o: In function `file_action':
bareboxenv.c:(.text+0x14): undefined reference to `xzalloc'
/tmp/cc9G2oOY.o: In function `concat_path_file':
bareboxenv.c:(.text+0xf4): undefined reference to `xmalloc'
/tmp/cc9G2oOY.o: In function `envfs_save':
bareboxenv.c:(.text+0x5b4): undefined reference to `xzalloc'
/tmp/cc9G2oOY.o: In function `envfs_load':
bareboxenv.c:(.text+0x7d8): undefined reference to `xmalloc'
collect2: error: ld returned 1 exit status

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>