pcu_l1_if: move direct phy l1if functions to dedicated header file

The function headers for the direct phy related functions (l1if_...) are
currently directly listed in pcu_l1_if.cpp and osmobts_sock.c. Those
functions are basically the API interface definition between the
platform independed PCU code and the platform dependant direct phy code.
Lets put them into a dedicated header file.

Change-Id: Id54b96a4cd498a9860f3f209498132455e2aa6f8
Related: OS#5198
diff --git a/src/pcu_l1_if.cpp b/src/pcu_l1_if.cpp
index 02f56ac..62f02d9 100644
--- a/src/pcu_l1_if.cpp
+++ b/src/pcu_l1_if.cpp
@@ -43,6 +43,7 @@
 #include <osmocom/gsm/gsm0502.h>
 
 #include <nacc_fsm.h>
+#include <pcu_l1_if_phy.h>
 }
 
 #include <gprs_rlcmac.h>
@@ -56,15 +57,6 @@
 #include <tbf_dl.h>
 #include <gprs_ms_storage.h>
 
-// FIXME: move this, when changed from c++ to c.
-extern "C" {
-void *l1if_open_pdch(uint8_t trx_no, uint32_t hlayer1,
-		     struct gsmtap_inst *gsmtap);
-int l1if_connect_pdch(void *obj, uint8_t ts);
-int l1if_pdch_req(void *obj, uint8_t ts, int is_ptcch, uint32_t fn,
-        uint16_t arfcn, uint8_t block_nr, uint8_t *data, uint8_t len);
-}
-
 extern void *tall_pcu_ctx;
 
 #define PAGING_GROUP_LEN 3