core: fix build warning on newer versions of gcc
    
    When building libusb on a "newer" version of gcc (9.2), a lot of
    warnings are thrown about zero-length messages as being part of a format
    string.
    
    An example of this is:
    
    descriptor.c:546:11: warning: zero-length gnu_printf format string [-Wformat-zero-length]
      546 |  usbi_dbg("");
          |           ^~
    
    Fix this up by replacing all calls of:
    	usbi_dbg("");
    with
    	usbi_dbg(" ");
    as obviously we still want to keep the implicit tracing message in the
    log.
    
    Closes #674
    
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>
    (cherry picked from commit 0bf84e4d516c4488e5fcf4b10e3a7263c13019e1)
    
    patch_name: 0007-core-fix-build-warning-on-newer-versions-of-gcc.patch
    present_in_specfile: true
    location_in_specfile: 7
    squash_commits: true