Add Win32/ to gitginore list (#3308)

Visual Studio .Net used Win32/ as one of the default output directories for C and C++ projects. Later, when 64-bit support was added to the toolchain (circa 2005), x64/ was used. The Gitignore files include x64/, but not Win32/. The commit adds support for both Win32/ and x64/.

VisualStudio.gitignore

@@ -23,6 +23,7 @@ mono_crash.*
 [Rr]eleases/
 x64/
 x86/
+[Ww][Ii][Nn]32/
 [Aa][Rr][Mm]/
 [Aa][Rr][Mm]64/
 bld/