libapparmor: fix reallocarray FTBFS w/older glibc

The recently added overlay cache directory support added to libapparmor makes use of reallocarray(3) to resize memory allocations; however, reallocarray() was only included in glibc 2.26. This commit adds a configure check for reallocarray() and if it's not available, provides it as a wrapper around realloc(3).

Signed-off-by: Steve Beattie steve.beattie@canonical.com