perf tools: Enable strict JSON parsing

This is to ensure that the PMU event files can always be parsed by
other tools.

Testing
=======

 * There are no errors when parsing files for all architectures:
     # pmu-events/jevents nds32 pmu-events/arch/ test
     # pmu-events/jevents s390 pmu-events/arch/ test
     # pmu-events/jevents powerpc pmu-events/arch/ test
     # pmu-events/jevents arm64 pmu-events/arch/ test
     # pmu-events/jevents test pmu-events/arch/ test
     # pmu-events/jevents x86 pmu-events/arch/ test

 * Trailing and leading commas now cause a parse error

 * Double commas now cause a parse error

 * Compilation and parsing works with strict mode disabled and enabled

 * A diff of the output files shows no changes

Signed-off-by: James Clark <james.clark@arm.com>