nsis: Correct building on NSIS 3.0

LVM_GETITEMCOUNT and LVM_GETITEMTEXT are already defined post NSIS 3.0.

makepanda/installer.nsi

@@ -1239,8 +1239,13 @@ done:
 
 FunctionEnd
 
+!ifndef LVM_GETITEMCOUNT
 !define LVM_GETITEMCOUNT 0x1004
+!endif
+
+!ifndef LVM_GETITEMTEXT
 !define LVM_GETITEMTEXT 0x102D
+!endif
 
 Function DumpLog
   Exch $5