ghdl: trim whitespace (#720)

Before:

    ghdl:  no declaration for "unsigned"

After:

    ghdl: no declaration for "unsigned"

Also, use (%l+) instead of ([^:]+) as severity seems to be always lowercase
error or warning.

lua/lint/linters/ghdl.lua

@@ -1,4 +1,4 @@
-local pattern = "([^:]+):(%d+):(%d+):([^:]+):(.+)"
+local pattern = "([^:]+):(%d+):(%d+):(%l+):%s*(.+)"
 local groups = { "file", "lnum", "col", "severity", "message" }
 local severity_map = {
   ["error"] = vim.diagnostic.severity.ERROR,