[Debug] declare LOG_STR_LAMBDA's return type because gcc 8.3 can't infer it

Current master does not build on our big boxes because gcc 8.3 can't infere the return type of our LOG_STR_LAMBDA and treats is as a void expression.

emper/Debug.hpp

@@ -18,7 +18,7 @@
 // The lamda should be fairly cheap/free because it will be inlined.
 // The use of a lambda here is the ISO C++ equivalent to GCC statement expressions.
 // NOLINTNEXTLINE(bugprone-macro-parentheses)
-#define LOG_STR_LAMBDA(x, new_line) [&](){std::stringstream sst; sst << x; if constexpr (new_line) { sst << std::endl; }; return sst.str(); }
+#define LOG_STR_LAMBDA(x, new_line) [&]() -> std::string {std::stringstream sst; sst << x; if constexpr (new_line) { sst << std::endl; }; return sst.str(); }
 
 // NOLINTNEXTLINE(bugprone-macro-parentheses)
 #define LOG(level, x, log_func, new_line) do {if constexpr (level > EMPER_LOG_LEVEL) { break; } log_func(LOG_STR_LAMBDA(x, new_line)()); } while (false)