Statically link winpthread

* src/Makefile.am (gpgol_LDFLAGS): Add -static -lpthread

As with libstdc++ and with libgcc_sjlj, this links in the
pthread library from the build system so that GpgOL has no
external dependencies again. Which makes it more robust to
replace the DLL as a patch in a different build.

• GnuPG-Bug-Id: T7241