Add 'void' to function definition to avoid compiler warnings

During compiling of FreeIPA we are receiving compiler error:

slapi-plugin.h:6149, GNU C Compiler 4 (gcc), Priority: Normal
function declaration isn't a prototype [-Wstrict-prototypes]

This patch should fix the compiler warning.

Reviewed by [email protected].

ldap/servers/slapd/slapi-plugin.h

@@ -6146,7 +6146,7 @@ int slapi_rwlock_unlock( Slapi_RWLock *rwlock );
  *
  * \return the size of Slapi_RWLock
  */
-int slapi_rwlock_get_size();
+int slapi_rwlock_get_size( void );
 
 /*
  * thread-safe LDAP connections

ldap/servers/slapd/slapi2nspr.c

@@ -261,7 +261,7 @@ slapi_rwlock_unlock( Slapi_RWLock *rwlock )
 }
 
 int
-slapi_rwlock_get_size()
+slapi_rwlock_get_size( void )
 {
 #ifdef USE_POSIX_RWLOCKS
     return sizeof(pthread_rwlock_t);