west: commands: Make functions that don't use 'self' static

Fixes this pylint warning:

    R0201: Method could be a function (no-self-use)

Another option would be to turn them into regular functions, but that'd
be a bigger change.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
diff --git a/scripts/west_commands/zephyr_ext_common.py b/scripts/west_commands/zephyr_ext_common.py
index 9ddfd82..fa6dcde 100644
--- a/scripts/west_commands/zephyr_ext_common.py
+++ b/scripts/west_commands/zephyr_ext_common.py
@@ -17,7 +17,8 @@
 class Forceable(WestCommand):
     '''WestCommand subclass for commands with a --force option.'''
 
-    def add_force_arg(self, parser):
+    @staticmethod
+    def add_force_arg(parser):
         '''Add a -f / --force option to the parser.'''
         parser.add_argument('-f', '--force', action='store_true',
                             help='Ignore any errors and try to proceed')