insserv: Fix build with gcc5 and clang

Kill 2 bugs with one patch

| insserv.c:997:5: note: each undeclared identifier is reported only
once for each function it appears in
| insserv.c:997:15: error: expected ';' before 'char'
|      extension char buf[strlen(myname)+2+strlen(fmt)+1];
|                ^
| insserv.c: In function 'main':
| insserv.c:2379:5: error: 'extension' undeclared (first use in this
function)
|      extension char * argr[argc];
|      ^
| insserv.c:2379:15: error: expected ';' before 'char'
|      extension char * argr[argc];
|                ^
| insserv.c:2401:2: error: 'argr' undeclared (first use in this
function)
|   argr[c] = (char*)0;
|   ^

Change-Id: I36b7fb9e8baeda5a7cc252da10c0527401248226
Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>