common: Add helper function to convert RSSI to RCPI

This conversion will be done several times in the code, so add a helper
function that does this conversion.

Signed-off-by: Avrahams Stern <avraham.stern@intel.com>

src/utils/common.h

@@ -552,6 +552,7 @@ int is_ctrl_char(char c);
 
 int str_starts(const char *str, const char *start);
 
+u8 rssi_to_rcpi(int rssi);
 
 /*
  * gcc 4.4 ends up generating strict-aliasing warnings about some very common