drivers: usb: buf: Do not add empty library

Change CMakeLists.txt to register Zephyr library only if respective
source code is compiled.

Fixes CMake Warning:
  No SOURCES given to Zephyr library: drivers__usb__common__buf

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>

Author: tmon-nordic

drivers/usb/common/buf/CMakeLists.txt

@@ -1,5 +1,7 @@
 # Copyright Nordic Semiconductor ASA
 # SPDX-License-Identifier: Apache-2.0
 
-zephyr_library()
-zephyr_library_sources_ifdef(CONFIG_UDC_DRIVER usb_buf.c)
+if(CONFIG_UDC_DRIVER OR CONFIG_UHC_DRIVER)
+  zephyr_library()
+  zephyr_library_sources(usb_buf.c)
+endif()