dieharder: add -fcommon workaround
dieharder: add -fcommon workaround

Workaround build failure on -fno-common toolchains like upstream
gcc-10. Otherwise build fails as:

    ld: include/dieharder/parse.h:21: multiple definition of `splitbuf';
      include/dieharder/parse.h:21: first defined here

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