json-c: avoid ptest failure caused by valgrind

json-c's ptest will automatically detect valgrind and use it to run
tests. This will fail in case valgrind does not work, e.g., intel-x86-64
(corei7-64) + lib32 image. The error is about illegal instruction.
Error message is like below:

  Illegal instruction     (core dumped) valgrind --tool=memcheck ...
  ERROR: "test1Formatted spaced pretty" exited with non-zero exit status: 132

So we explicitly disable valgrind by exporting USE_VALGRIND=0.

(From OE-Core rev: 78160463259ac3be6de380d12b8cd6159162792b)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b0429a278e39b7a2846144bcc3fcc24ca298cab7)
Signed-off-by: Steve Sakoman <steve@sakoman.com>

meta/recipes-devtools/json-c/json-c/run-ptest

@@ -3,6 +3,8 @@
 # This script is used to run json-c test suites
 cd tests
 
+export USE_VALGRIND=0
+
 ret_val=0
 for i in test*.test; do
     # test_basic is not an own testcase, just