helper: Fix compilation warning

Adding vk_enum_string_helper.h produced a warning that
GetPhysDevFeatureString was unused.  Made it inline (as all
the other commands are in that header) and it made the compiler
happy.

Change-Id: I9452ddfc8de4af4c88b95e380b9520d9f59796da
diff --git a/scripts/helper_file_generator.py b/scripts/helper_file_generator.py
index 2cbcf8e..7e0876d 100644
--- a/scripts/helper_file_generator.py
+++ b/scripts/helper_file_generator.py
@@ -388,7 +388,7 @@
                 pdev_members = members
                 break
         deindex = '\n'
-        deindex += 'static const char * GetPhysDevFeatureString(uint32_t index) {\n'
+        deindex += 'static inline const char * GetPhysDevFeatureString(uint32_t index) {\n'
         deindex += '    const char * IndexToPhysDevFeatureString[] = {\n'
         for feature in pdev_members:
             deindex += '        "%s",\n' % feature.name