.gitignore: ignore files generated by asn1 compiler

As described in doc/README.asn1 document the tools/asn1_compiler is used
to "generate bytecode as a C file (*.asn1.[ch]) from *.asn1 file".

Signed-off-by: Dario Binacchi <dariobin@libero.it>

.gitignore

@@ -7,6 +7,7 @@
 #
 .*
 *.a
+*.asn1.[ch]
 *.bin
 *.cfgout
 *.dtb