libusbg: Add empty definitions of gadget schemes functions
libusbg: Add empty definitions of gadget schemes functions

In some cases user may want to not use gadget schemes
functionality so he should be able to remove libconfig
dependency. To be able to do this without API break
add empty definitions of gadget schemes functions which
only returns USBG_ERROR_NOT_SUPPORTED.

Signed-off-by: Krzysztof Opasiak <8064eda6f94b3817252b7d7649cf754592f63500@samsung.com>

pszewczyk/libusbg,pszewczyk/libusbg