package/flex: disable reallocarray

package/flex: disable reallocarray

This doesn't compile with glibc 2.26 where reallocarray() as been introduced.

It's a nasty issue, when reallocarray() is available for the target, flex will
build a small tool called stage1flex for the host (using _FOR_BUILD) but with
the config.h generated for the target.

When the host doesn't have a glibc >= 2.26, reallocarray() is never defined
while building stage1flex:

misc.c:147:8: warning : implicit declaration of function « reallocarray »
[-Wimplicit-function-declaration]
  mem = reallocarray(NULL, (size_t) size, element_size);
        ^~~~~~~~~~~~
misc.c:147:6: warning : assignment makes pointer from integer without a cast
[-Wint-conversion]
  mem = reallocarray(NULL, (size_t) size, element_size);
      ^

Disable reallocarray for now, reallocarray() support may be
enabled in a followup patch.

Fixes:
stage1flex-misc.o: In function `allocate_array':
misc.c:(.text+0x38f): undefined reference to `reallocarray'
stage1flex-misc.o: In function `reallocate_array':
misc.c:(.text+0xc8a): undefined reference to `reallocarray'
collect2: error: ld returned 1 exit status

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>