testsuite: ctf: make array in ctf-file-scope-1 fixed length

The array member of struct SFOO in the ctf-file-scope-1 caused the test
to fail for the BPF target, since BPF does not support dynamic stack
allocation. The array does not need to variable length for the sake of
the test, so make it fixed length instead to allow the test to run
successfully for the bpf-unknown-none target.

gcc/testsuite/

	* gcc.dg/debug/ctf/ctf-file-scope-1.c (SFOO): Make array member
	fixed-length.

gcc/testsuite/gcc.dg/debug/ctf/ctf-file-scope-1.c

@@ -9,7 +9,7 @@
 
 int foo (int n)
 {
-  typedef struct { int a[n]; } SFOO;
+  typedef struct { int a[6]; } SFOO;
 
   SFOO a;
   __attribute__ ((noinline)) SFOO gfoo (void) { return a; }