perf llvm-utils: Remove bashism from kernel include fetch script

perf llvm-utils: Remove bashism from kernel include fetch script

Like system(), popen() calls /bin/sh, which may/may not be bash.

Script when run on dash and encounters the line, yields:

 exit: Illegal number: -1

checkbashisms report on script content:

 possible bashism (exit|return with negative status code):
 exit -1

Remove the bashism and use the more portable non-zero failure
status code 1.

Signed-off-by: Kim Phillips <kim.phillips@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandipan Das <sandipan@linux.vnet.ibm.com>
Cc: Thomas Richter <tmricht@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20180629124652.8d0af7e2281fd3fd8262cacc@arm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>