Update header includes

Many files include unnecessary headers and don't include headers which
are actually used. Because of that combined with the fact that OsmoPCU
is a mixture of C and C++, it makes it hard to modularize code. Fix
this (using iwyu [1] tool):

* add missing headers
* remove unused headers

[1] https://include-what-you-use.org/

Related: OS#1539
Change-Id: I8c9f488a43b099c72b2d30d3245e7ba50872fc00
diff --git a/src/rlc.cpp b/src/rlc.cpp
index d7f0609..a82f550 100644
--- a/src/rlc.cpp
+++ b/src/rlc.cpp
@@ -18,11 +18,17 @@
 
 #include "bts.h"
 #include "gprs_debug.h"
+#include <gprs_coding_scheme.h>
+#include <rlc.h>
 
-#include <errno.h>
+#include <stdbool.h>
+#include <string.h>
+#include <sys/types.h>
 
 extern "C" {
 #include <osmocom/core/utils.h>
+#include <osmocom/core/bitvec.h>
+#include <osmocom/core/logging.h>
 }