LibRegex: Allow the SingleMatch flag to be used as a PosixFlag

Userland/Libraries/LibRegex/RegexOptions.h

@@ -53,6 +53,7 @@ enum class PosixFlags : FlagsUnderlyingType {
     SkipSubExprResults = (FlagsUnderlyingType)AllFlags::SkipSubExprResults,
     SkipTrimEmptyMatches = (FlagsUnderlyingType)AllFlags::SkipTrimEmptyMatches,
     Multiline = (FlagsUnderlyingType)AllFlags::Multiline,
+    SingleMatch = (FlagsUnderlyingType)AllFlags::SingleMatch,
     StringCopyMatches = (FlagsUnderlyingType)AllFlags::StringCopyMatches,
 };