zdump: constify dfi_vmcoreinfo_get

Caller of `dfi_vmcoreinfo_get` must not modify the returned value, therefore
return a `const char *` value for `dfi_vmcoreinfo_get`.

Signed-off-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

zdump/dfi_vmcoreinfo.c

@@ -132,7 +132,7 @@ void dfi_vmcoreinfo_init(void)
 /*
  * Return vmcoreinfo data
  */
-char *dfi_vmcoreinfo_get(void)
+const char *dfi_vmcoreinfo_get(void)
 {
 	return l.vmcoreinfo;
 }