advpack: Constify a variable

[Andrew Talbot]

Changelog:
    advpack: Constify a variable.

diff --git a/dlls/advpack/install.c b/dlls/advpack/install.c
index 129ea82..d3d31ed 100644
--- a/dlls/advpack/install.c
+++ b/dlls/advpack/install.c
@@ -258,7 +258,7 @@ static WCHAR *get_field_string(INFCONTEXT *context, DWORD index, WCHAR *buffer,
 
 /* iterates over all fields of a certain key of a certain section */
 static HRESULT iterate_section_fields(HINF hinf, PCWSTR section, PCWSTR key,
-                                      iterate_fields_func callback, void *arg)
+                                      iterate_fields_func callback, const void *arg)
 {
     WCHAR static_buffer[200];
     WCHAR *buffer = static_buffer;