Add a missing extern, to fix building with -fno-common
With C compilers defaulting to -fcommon, this isn't an issue, but upcoming compilers (GCC 10 and Clang 11) will default to -fno-common, ending up with duplicate definitions of these variables.
With C compilers defaulting to -fcommon, this isn't an issue, but upcoming compilers (GCC 10 and Clang 11) will default to -fno-common, ending up with duplicate definitions of these variables.