libssp: Include 'stdlib.h' for using alloca(3) [PR116547]

As GCC now treating implicit declaration of function as an error instead of
warning, compilation of libssp has been broken on some operating systems.
The following error is from an x86_64-unknown-freebsd11 system:

    [...]/libssp/ssp.c: In function 'fail':
    [...]/libssp/ssp.c:134:17: error: implicit declaration of function 'alloca' [-Wimplicit-function-declaration]
      134 |       p = buf = alloca (len);
          |                 ^~~~~~

Similarly: amd64-unknown-openbsd7.9.

Most operating systems specifies that 'stdlib.h' should be included to get
the declaration of alloca(3).

	PR other/116547
	* ssp.c: Include stdlib.h for alloca(3).

Co-authored-by: Thomas Schwinge <tschwinge@baylibre.com>