Consider creating an API namespace for `libar-gpr`

[slyon]

The libar-gpr API currently installs directly into usr/include and does not have any consistent prefix. We should consider installing those files into usr/include/gpr (and updating the corresponding pkg-config file accordingly).

-rw-r--r-- root/root      2397 2026-09-07 13:52 ./usr/include/ar_guids.h
-rw-r--r-- root/root       543 2026-09-07 13:52 ./usr/include/ar_types.h
-rw-r--r-- root/root      9878 2026-09-07 13:52 ./usr/include/gpr_api.h
-rw-r--r-- root/root     18520 2026-09-07 13:52 ./usr/include/gpr_api_i.h
[...]
-rw-r--r-- root/root      8976 2026-09-07 13:52 ./usr/include/ipc_dl_api.h

We should avoid mixed prefix (ar_, gpr_, ipc_), without installing them into a common directory.

Other libraries from this project, install into usr/include directly, too. But they share a common prefix for namespaceing, e.g.:

-rw-r--r-- root/root     13296 2026-09-07 13:52 ./usr/include/spf/aac_decoder_api.h
-rw-r--r-- root/root     16338 2026-09-07 13:52 ./usr/include/spf/aac_encoder_api.h
-rw-r--r-- root/root     17765 2026-09-07 13:52 ./usr/include/spf/acd_api.h
[...]
-rw-r--r-- root/root     72588 2026-09-07 13:52 ./usr/include/acdb.h
-rw-r--r-- root/root      1232 2026-09-07 13:52 ./usr/include/acdb_begin_pack.h
-rw-r--r-- root/root      4979 2026-09-07 13:52 ./usr/include/acdb_command.h
[...]
-rw-r--r-- root/root      9991 2026-09-07 13:52 ./usr/include/gsl_common.h
-rw-r--r-- root/root      2677 2026-09-07 13:52 ./usr/include/gsl_cshm_mgr.h
-rw-r--r-- root/root      2358 2026-09-07 13:52 ./usr/include/gsl_dls_client.h
[...]
-rw-r--r-- root/root      3059 2026-09-07 13:52 ./usr/include/ar_osal_error.h
-rw-r--r-- root/root      6017 2026-09-07 13:52 ./usr/include/ar_osal_file_io.h
-rw-r--r-- root/root      4142 2026-09-07 13:52 ./usr/include/ar_osal_heap.h