filesystem MAINTENANCE move public ly_ctx* functions into context.h

Private definition of the ly_ctx structure is not in common.h header
file and all the context's public functions will be declared in
context.h.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f15b60f..d06d2e6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -178,6 +178,7 @@
 
 set(headers
     src/libyang.h
+    src/context.h
     src/tree_schema.h
     src/dict.h
     src/log.h