symbols-check: Fix symbol demangling for Windows

Only strip leading underscores if there's also a trailing @
Fixes shared-glapi symbol check for x64

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12881>

bin/symbols-check.py

@@ -73,7 +73,7 @@ def get_symbols_dumpbin(dumpbin, lib):
             continue
         symbol_name = fields[3]
         # De-mangle symbols
-        if symbol_name[0] == '_':
+        if symbol_name[0] == '_' and '@' in symbol_name:
             symbol_name = symbol_name[1:].split('@')[0]
         symbols.append(symbol_name)
     return symbols