all: fix additional compiler warningis about function declarations

warning: function declaration isn’t a prototype [-Wstrict-prototypes]

In C function() and function(void) are two different prototypes (as opposed to
C++).
function()     accepts an arbitrary number of arguments
function(void) accepts zero arguments

libnm-core/tests/test-general.c

@@ -4395,7 +4395,7 @@ enum TEST_IS_POWER_OF_TWP_ENUM_UNSIGNED_64 {
 	} G_STMT_END
 
 static void
-test_nm_utils_is_power_of_two ()
+test_nm_utils_is_power_of_two (void)
 {
 	guint64 xyes, xno;
 	gint i, j;

src/nm-multi-index.c

@@ -42,7 +42,7 @@ typedef struct {
 /******************************************************************************************/
 
 static ValuesData *
-_values_data_create ()
+_values_data_create (void)
 {
 	ValuesData *values_data;
 

src/platform/tests/test-general.c

@@ -31,7 +31,7 @@
 /******************************************************************/
 
 static void
-test_init_linux_platform ()
+test_init_linux_platform (void)
 {
 	gs_unref_object NMPlatform *platform = NULL;
 

src/platform/tests/test-nmp-object.c

@@ -212,7 +212,7 @@ static const NMPlatformLink pl_link_3 = {
 };
 
 static void
-test_cache_link ()
+test_cache_link (void)
 {
 	NMPCache *cache;
 	NMPObject *obj1, *obj2;