pylibfdt: Rework "avoid unused variable warning" lines

Clang has -Wself-assign enabled by default under -Wall and so when
building with -Werror we would get an error here.  Inspired by Linux
kernel git commit a21151b9d81a ("tools/build: tweak unused value
workaround") make use of the fact that both Clang and GCC support
casting to `void` as the method to note that something is intentionally
unused.

Signed-off-by: Tom Rini <trini@konsulko.com>

scripts/dtc/pylibfdt/libfdt.i_shipped

@@ -1010,7 +1010,7 @@ typedef uint32_t fdt32_t;
 	}
 	$1 = (void *)PyByteArray_AsString($input);
         fdt = $1;
-        fdt = fdt; /* avoid unused variable warning */
+        (void)fdt; /* avoid unused variable warning */
 }
 
 /* Some functions do change the device tree, so use void * */
@@ -1021,7 +1021,7 @@ typedef uint32_t fdt32_t;
 	}
 	$1 = PyByteArray_AsString($input);
         fdt = $1;
-        fdt = fdt; /* avoid unused variable warning */
+        (void)fdt; /* avoid unused variable warning */
 }
 
 /* typemap used for fdt_get_property_by_offset() */