Set default compression for NSIS installer to LZMA.

Use of LZMA (versus the current zlib) shaves a few MB off the installer.

share/setup.nsi

@@ -1,6 +1,7 @@
 Name Bitcoin
 
 RequestExecutionLevel highest
+SetCompressor /SOLID lzma
 
 # General Symbol Definitions
 !define REGKEY "SOFTWARE\$(^Name)"