performance: declare gumbo_debug as inline function

[detailyang]

Now using the perf profiler on user space to benchmark gumbo_parser at Centos6.5 as the following:

perf record -g --all-user benchmarks/benchmark

It shows the gumbo_debug function will occupy approximate %1.10 On-CPU at retq instruction.

and

To eliminate the overhead of function call even if it's compiled on release mode, gumbo_debug should be declared as inline function.

Also apply @stevecheckoway's PR #378 to pass CI.

@nostrademons Can you have a look at it ? Many thanks !

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