Clean up various C source and headers to appease -Wstrict-prototypes

In C, func() is a function taking an unspecified number of arguments,
vs func(void) a function taking no arguments. In C++ both forms indicate
"no arguments."

Update these headers to use the (void) form, which is correct in both
languages and avoids complaints when -Wstrict-prototypes is specified.

test/kitchen_sink/CMakeLists.txt

@@ -59,6 +59,7 @@ target_compile_options(kitchen_sink_options INTERFACE
         -Wfloat-equal
         -Wmissing-format-attribute
         -Wno-long-long
+        $<$<COMPILE_LANGUAGE:C>:-Wstrict-prototypes>
 
         # todo not sure these are true, but investigate
         #-Wpacked

test/kitchen_sink/kitchen_sink.c

@@ -39,7 +39,7 @@ uint32_t *foo = (uint32_t *) 200;
 uint32_t dma_to = 0;
 uint32_t dma_from = 0xaaaa5555;
 
-void spiggle() {
+void spiggle(void) {
     dma_channel_config c = dma_channel_get_default_config(1);
     channel_config_set_bswap(&c, true);
     channel_config_set_transfer_data_size(&c, DMA_SIZE_16);
@@ -48,7 +48,7 @@ void spiggle() {
     dma_channel_transfer_from_buffer_now(1, foo, 23);
 }
 
-void __isr dma_handler_a() {
+void __isr dma_handler_a(void) {
     printf("HELLO A\n");
     if (dma_hw->ints1 & 1) {
         dma_hw->ints1 = 1;
@@ -57,7 +57,7 @@ void __isr dma_handler_a() {
     }
 }
 
-void __isr dma_handler_b() {
+void __isr dma_handler_b(void) {
     printf("HELLO B\n");
     if (dma_hw->ints1 & 1) {
         dma_hw->ints1 = 1;
@@ -68,7 +68,7 @@ void __isr dma_handler_b() {
 
 //#pragma GCC pop_options
 
-int main() {
+int main(void) {
     spiggle();
 
     stdio_init_all();