Revert "remove constexpr on level_string_views to fix compilation on C++17 from addition of set_string_view"

This reverts commit ac3e26b0ff.

include/spdlog/common-inl.h

@@ -13,6 +13,9 @@
 namespace spdlog {
 namespace level {
 
+#if __cplusplus >= 201703L
+constexpr
+#endif
 static string_view_t level_string_views[] SPDLOG_LEVEL_NAMES;
 
 static const char *short_level_names[] SPDLOG_SHORT_LEVEL_NAMES;