Include source root directory

All headers and sources are in src/. To avoid using relative includes
from subdirectories ("../../"), include the source root directory.

app/src/sys/win/command.c

@@ -1,7 +1,7 @@
-#include "../../command.h"
+#include "command.h"
 
-#include "../../log.h"
-#include "../../strutil.h"
+#include "log.h"
+#include "strutil.h"
 
 HANDLE cmd_execute(const char *path, const char *const argv[]) {
     STARTUPINFO si;