ada: Fix Ada bootstrap on Solaris

ada: Fix Ada bootstrap on Solaris

The recent warning patches broke Ada bootstrap on Solaris:

adaint.c: In function '__gnat_kill':
adaint.c:3597:3: error: implicit declaration of function 'kill'
[-Wimplicit-function-declaration]
 3597 |   kill (pid, sig);
      |   ^~~~

expect.c: In function '__gnat_expect_poll':
expect.c:409:5: error: implicit declaration of function 'memset'
[-Wimplicit-function-declaration]
  409 |     FD_ZERO (&rset);
      |     ^~~~~~~
expect.c:55:1: note: include '<string.h>' or provide a declaration of 'memset'
   54 | #include <sys/wait.h>
  +++ |+#include <string.h>
   55 | #endif

I'm now including the necessary headers: <signal.h> for kill and
<string.h> for memset.

Bootstrapped without regressions on i386-pc-solaris2.11,
sparc-sun-solaris2.11, x86_64-pc-linux-gnu, and
x86_64-apple-darwin23.1.0.

2023-12-03  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/ada:
	* adaint.c: Include <signal.h>.
	* expect.c: Include <string.h>.