debian: Install header and source file to different directory

osmo-bts consumes the gsm_data_shared.{h,c} files and assumes the
OpenBSC directory layout. Install the files into the specific
sub-directories to make the --with-openbsc=dir option work.

Change-Id: I3da21402dee3d6f1d812b232b0a3f13b8a462d5b
diff --git a/debian/control b/debian/control
index 93b4bd3..144e649 100644
--- a/debian/control
+++ b/debian/control
@@ -107,8 +107,8 @@
  from OpenBSC. This package installs these files to your file system so
  that the other packages can build-depend on this package.
  .
- The header file is installed to /usr/include/osmocom/openbsc, while
- the .c file is installed to /usr/src/osmocom/openbsc.
+ The directory structure is copied after the structure in the repository
+ and the header and .c file are installed into /usr/src/osmocom/openbsc/.
 
 Package: osmo-gtphub
 Architecture: any