mdbx: add pure- & const-function attributes to C API.

Change-Id: Ie4d1742f3d4717a0cd1fd5677b9b1ae641193d45

src/osal.h

@@ -511,7 +511,8 @@ MDBX_INTERNAL_VAR bool
 /* Get the size of a memory page for the system.
  * This is the basic size that the platform's memory manager uses, and is
  * fundamental to the use of memory-mapped files. */
-static __maybe_unused __inline size_t mdbx_syspagesize(void) {
+__nothrow_const_function static __maybe_unused __inline size_t
+mdbx_syspagesize(void) {
 #if defined(_WIN32) || defined(_WIN64)
   SYSTEM_INFO si;
   GetSystemInfo(&si);