fix: missing sstream header added to fixed_string_test

test/runtime/fixed_string_test.cpp

@@ -23,6 +23,7 @@
 #include <catch2/catch_test_macros.hpp>
 #include <catch2/matchers/catch_matchers_exception.hpp>
 #include <mp-units/compat_macros.h>
+#include <sstream>
 #include <string_view>
 #ifdef MP_UNITS_MODULES
 import mp_units;