[tools] Unbreak the GCC build (workaround a GCC bug).

../tools/llvm-extract/llvm-extract.cpp: In function ‘int main(int, char**)’:
warning: ISO C++ forbids zero-size array ‘argv’ [-Wpedantic]

GCC reference bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61259

llvm-svn: 286396