remove unnecessary semicolon

[jameslamb]

Looking at the logs from #629 (comment), I noticed many instances of this:

In file included from ./include/LightGBM/utils/common.h:9:0,
                 from ./include/LightGBM/config.h:16,
                 from io/config_auto.cpp:8:
./include/LightGBM/utils/openmp_wrapper.h:84:2: warning: extra ‘;’ [-Wpedantic]
 };  // extern "C"

Uses of extern "C" {} don't need a closing ;.

This PR proposes removing the trailing ; in openmp_wrapper.h to suppress this compiler warning when using -pedantic.

You can see the R Hub logs here: https://builder.r-hub.io/status/original/lightgbm_2.3.2.tar.gz-6d1aa84ffbbd409293c9dcd94714bb47

[github-actions]

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