Fix compilation with GCC13

See also https://bugs.debian.org/1037668

src/drm.hpp

@@ -93,6 +93,7 @@ extern "C" {
 #include <unordered_map>
 #include <mutex>
 #include <vector>
+#include <string>
 
 struct saved_mode {
 	int width;

src/main.cpp

@@ -5,6 +5,7 @@
 #include <mutex>
 #include <vector>
 #include <cstring>
+#include <string>
 #if defined(__linux__)
 #include <sys/capability.h>
 #endif

src/sdlwindow.cpp

@@ -2,6 +2,7 @@
 
 #include <thread>
 #include <mutex>
+#include <string>
 
 #include <linux/input-event-codes.h>