app/memcmp: fix compilation on Ubuntu 20.04.

Fixes:
main.c: In function ‘fault_handler’:
main.c:134:9: error: a label can only be part of a statement and a declaration
is not a statement

app/memcmp/main.c

@@ -99,6 +99,8 @@ void aep_cb_func(void)
 /* Called upon SIGSEGV caused by untrusted page tables. */
 void fault_handler(int signo, siginfo_t * si, void  *ctx)
 {
+    ucontext_t *uc = (ucontext_t *) ctx;
+
     switch ( signo )
     {
       case SIGSEGV:
@@ -131,7 +133,6 @@ void fault_handler(int signo, siginfo_t * si, void  *ctx)
         #endif
 
         /* ensure RFLAGS.TF is clear to disable debug single-stepping */
-        ucontext_t *uc = (ucontext_t *) ctx;
         uc->uc_mcontext.gregs[REG_EFL] &= ~0x100;
         break;
     #endif