pcr: libosmocore: fix libremakepkg build due to missing libusb
    
    Without that fix it builds fine with makepkg if you have libusb
    installed but it fails with libremakepkg with the following error:
     |  checking for LIBUSB... no
     |  configure: error: Package requirements (libusb-1.0) were not met:
     |
     |  Package 'libusb-1.0', required by 'virtual:world', not found
     |
     |  Consider adjusting the PKG_CONFIG_PATH environment variable if you
     |  installed software in a non-standard prefix.
     |
     |  Alternatively, you may set the environment variables LIBUSB_CFLAGS
     |  and LIBUSB_LIBS to avoid the need to call pkg-config.
     |  See the pkg-config man page for more details.
    
    Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>