codec: Add the missing bits for include to install and pkgconfig to work

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/include/osmocom/codec/Makefile.am b/include/osmocom/codec/Makefile.am
new file mode 100644
index 0000000..c213602
--- /dev/null
+++ b/include/osmocom/codec/Makefile.am
@@ -0,0 +1,3 @@
+osmocodec_HEADERS = codec.h
+
+osmocodecdir = $(includedir)/osmocom/codec