conv_gen: Use python2 to execute the script

The script does not work with python3:

$ python3 utils/conv_gen.py
  File "utils/conv_gen.py", line 124
    def _print_term(self, fi, num_states, pack = False):

Second there is no 'python' on FreeBSD and one needs to select
the major version to use.

  GEN      conv_cs3_gen.c
  GEN      conv_xcch_gen.c
  GEN      conv_cs2_gen.c
python: not found
python: not found
python: not found

By using python2 we solve both issues. On Debian python2 is located
inside the python-minimal package.
diff --git a/src/gsm/Makefile.am b/src/gsm/Makefile.am
index 1b186b1..7f5fa25 100644
--- a/src/gsm/Makefile.am
+++ b/src/gsm/Makefile.am
@@ -37,4 +37,4 @@
 EXTRA_DIST = libosmogsm.map
 
 conv%gen.c: ../../utils/conv_gen.py
-	$(AM_V_GEN)python ../../utils/conv_gen.py
+	$(AM_V_GEN)python2 ../../utils/conv_gen.py
diff --git a/utils/conv_gen.py b/utils/conv_gen.py
index 1377fca..bb547de 100644
--- a/utils/conv_gen.py
+++ b/utils/conv_gen.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/python2
 
 mod_license = """
 /*