gsm0503.h: generate header automatically

Since we have automatic header generation, implemented in the
utils/conv_gen.py, it's time to use this feature!

Change-Id: I21caa4e433b2cc1861611e35350a9671da444c2a
diff --git a/.gitignore b/.gitignore
index 48642ca..84e11b8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -117,6 +117,7 @@
 src/gsm/gsm0503_conv.c
 include/osmocom/core/crc*gen.h
 include/osmocom/core/bit*gen.h
+include/osmocom/gsm/gsm0503.h
 
 # vi files
 *.sw?