[nostalgia/core] Replace old namespace nesting with C++17 nesting

src/nostalgia/core/userland/media.cpp

@@ -11,12 +11,10 @@
 
 #include "../media.hpp"
 
-namespace nostalgia {
-namespace core {
+namespace nostalgia::core {
 
 uint8_t *loadRom(const char*) {
 	return nullptr;
 }
 
 }
-}

src/nostalgia/core/userland/mem.cpp

@@ -6,11 +6,9 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  */
 
-namespace nostalgia {
-namespace core {
+namespace nostalgia::core {
 
 void initHeap() {
 }
 
 }
-}