dbghelp: Constify some variables

[Andrew Talbot]

Changelog:
    dbghelp: Constify some variables.

diff --git a/dlls/dbghelp/minidump.c b/dlls/dbghelp/minidump.c
index 7c624c9..a786d29 100644
--- a/dlls/dbghelp/minidump.c
+++ b/dlls/dbghelp/minidump.c
@@ -428,7 +428,7 @@ static void writeat(struct dump_context* dc, RVA rva, const void* data, unsigned
  * writes a new chunk of data to the minidump, increasing the current
  * rva in dc
  */
-static void append(struct dump_context* dc, void* data, unsigned size)
+static void append(struct dump_context* dc, const void* data, unsigned size)
 {
     writeat(dc, dc->rva, data, size);
     dc->rva += size;
diff --git a/dlls/dbghelp/pe_module.c b/dlls/dbghelp/pe_module.c
index 91c7d80..44a4d6a 100644
--- a/dlls/dbghelp/pe_module.c
+++ b/dlls/dbghelp/pe_module.c
@@ -134,7 +134,7 @@ static BOOL pe_load_dbg_file(const struct process* pcs, struct module* module,
  */
 static BOOL pe_load_msc_debug_info(const struct process* pcs, 
                                    struct module* module,
-                                   void* mapping, IMAGE_NT_HEADERS* nth)
+                                   void* mapping, const IMAGE_NT_HEADERS* nth)
 {
     BOOL                        ret = FALSE;
     const IMAGE_DATA_DIRECTORY* dir;
@@ -182,7 +182,7 @@ static BOOL pe_load_msc_debug_info(const struct process* pcs,
  */
 static BOOL pe_load_export_debug_info(const struct process* pcs, 
                                       struct module* module, 
-                                      void* mapping, IMAGE_NT_HEADERS* nth)
+                                      void* mapping, const IMAGE_NT_HEADERS* nth)
 {
     unsigned int 		        i;
     const IMAGE_EXPORT_DIRECTORY* 	exports;