shell32: Constify a variable

[Andrew Talbot]

I believe this exported function is undocumented, so it is all right to
constify its parameter.

-- Andy.
---
Changelog:
    shell32: Constify a variable.

diff -urN a/dlls/shell32/shell.c b/dlls/shell32/shell.c
--- a/dlls/shell32/shell.c	2007-09-02 11:02:16.000000000 +0100
+++ b/dlls/shell32/shell.c	2007-09-11 22:08:16.000000000 +0100
@@ -307,7 +307,7 @@
 
 /**********************************************************************/
 
-SEGPTR WINAPI FindEnvironmentString16(LPSTR str)
+SEGPTR WINAPI FindEnvironmentString16(LPCSTR str)
 { SEGPTR  spEnv;
   LPSTR lpEnv,lpString;
   TRACE("\n");