m68k: fix file path comment

Remove the wrong full path name of this file.
Signed-off-by: NGreg Ungerer <gerg@linux-m68k.org>

arch/m68k/include/asm/flat.h

 /*
- * include/asm-m68knommu/flat.h -- uClinux flat-format executables
+ * flat.h -- uClinux flat-format executables
  */
 
 #ifndef __M68KNOMMU_FLAT_H__