selftests/powerpc: Fix to use ucontext_t instead of struct ucontext

With glibc 2.26 'struct ucontext' is removed to improve POSIX
compliance, which breaks powerpc/alignment_handler selftest. Fix the
test by using ucontext_t. Tested on ppc, works with older glibc
versions as well.

Fixes the following:
  alignment_handler.c: In function ‘sighandler’:
  alignment_handler.c:68:5: error: dereferencing pointer to incomplete type ‘struct ucontext’
    ucp->uc_mcontext.gp_regs[PT_NIP] += 4;
Signed-off-by: Harish <harish@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/alignment/alignment_handler.c

@@ -57,7 +57,7 @@ volatile int gotsig;
 
 void sighandler(int sig, siginfo_t *info, void *ctx)
 {
-	struct ucontext *ucp = ctx;
+	ucontext_t *ucp = ctx;
 
 	if (!testing) {
 		signal(sig, SIG_DFL);