Warn the user if pkg-config isn't installed

Helpfully suggest installing pkg-config if FindLibUSB.cmake fails.
This change should not impact the build mechanics.

Old error:
```
rtucker@li583-148:~/bladerf/bladeRF/host/build$ cmake ..
-- The C compiler identification is GNU
-- Check for working C compiler: /usr/bin/gcc
-- Check for working C compiler: /usr/bin/gcc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Build type not specified: defaulting to a release build.
CMake Error at cmake/modules/FindLibUSB.cmake:121 (message):
  libusb-1.0 not found - is it installed? If you're using a binary
  distribution, try setting -DLIBUSB_PATH=<path_to_libusb_files>.
Call Stack (most recent call first):
  libraries/libbladeRF/CMakeLists.txt:99 (find_package)


-- Configuring incomplete, errors occurred!
```

New error:
```
rtucker@li583-148:~/bladerf/bladeRF/host/build$ cmake ..
-- Build type not specified: defaulting to a release build.
CMake Warning at cmake/modules/FindLibUSB.cmake:46 (message):
  Could not find pkg-config.  If libusb-1.0 can't be found, please install
  pkg-config.
Call Stack (most recent call first):
  libraries/libbladeRF/CMakeLists.txt:99 (find_package)


CMake Error at cmake/modules/FindLibUSB.cmake:125 (message):
  libusb-1.0 not found - is it installed? If you're using a binary
  distribution, try setting -DLIBUSB_PATH=<path_to_libusb_files>.  If
  pkg-config wasn't found, try installing it.
Call Stack (most recent call first):
  libraries/libbladeRF/CMakeLists.txt:99 (find_package)


-- Configuring incomplete, errors occurred!
```