adb: fix darwin build break.

Fix the following build breakage:

    system/core/adb/client/usb_osx.cpp:433:13: error: static declaration of 'usb_cleanup' follows non-static declaration
    static void usb_cleanup() NO_THREAD_SAFETY_ANALYSIS {
                ^
    system/core/adb/usb.h:48:5: note: previous declaration is here
        ADB_USB_INTERFACE(native::usb_handle*);

Bug: http://b/38203892
Test: mma on linux, darwin
Change-Id: I1c042e4d64ddcc0713f329202bc2037e590a9b4f