need to ensure openlibm static library gets installed

Due to `UNTRUSTED_SYSTEM_LIBM` flag on Windows, `libopenlibm.a` gets linked into `julia.exe` in `ui/Makefile`. Without this change, situations where `usr/bin/libopenlibm.dll` is present but `usr/lib/libopenlibm.a` is not can cause a link failure.