Warn on implicit switch case fallthrough

This seems to have found one actual bug in fs-sink.cc: the symlink case
was falling into the regular file case, which can't possibly be
intentional, right?

src/libexpr/lexer.l

@@ -94,6 +94,9 @@ static StringToken unescapeStr(SymbolTable & symbols, char * s, size_t length)
 
 }
 
+// yacc generates code that uses unannotated fallthrough.
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+
 #define YY_USER_INIT initLoc(yylloc)
 #define YY_USER_ACTION adjustLoc(yylloc, yytext, yyleng);