fs: add missing <cassert> include

This is needed to prevent compilation failures once boost is removed,
however is still correct to include now, and reduces the diff in #20744.

<string> is extracted from the defines because it is used for Windows
and non-Windows code, i.e get_filesystem_error_message().

src/fs.cpp

@@ -7,7 +7,6 @@
 #ifndef WIN32
 #include <cstring>
 #include <fcntl.h>
-#include <string>
 #include <sys/file.h>
 #include <sys/utsname.h>
 #include <unistd.h>
@@ -20,6 +19,9 @@
 #include <windows.h>
 #endif
 
+#include <cassert>
+#include <string>
+
 namespace fsbridge {
 
 FILE *fopen(const fs::path& p, const char *mode)