Add sftp library to sbbs3 MSVC solution

I'm not clear on why MSBuild can build the solution without this but the IDE
cannot (results in sftp.lib not found, unresolved SFTP symbols, etc.).

src/sbbs3/sbbs3.sln

@@ -100,6 +100,8 @@ Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "conio_sdl", "..\conio\conio
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "conio_gdi", "..\conio\conio_gdi.vcxproj", "{FBCCE03B-65EE-44A1-BBE1-B1F789D223A1}"
 EndProject
+Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "sftp", "..\sftp\sftp.vcxproj", "{4C69A1D2-182A-45D3-AD33-54A8A91B387D}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Win32 = Debug|Win32
@@ -269,6 +271,10 @@ Global
 		{FBCCE03B-65EE-44A1-BBE1-B1F789D223A1}.Debug|Win32.Build.0 = Debug|Win32
 		{FBCCE03B-65EE-44A1-BBE1-B1F789D223A1}.Release|Win32.ActiveCfg = Release|Win32
 		{FBCCE03B-65EE-44A1-BBE1-B1F789D223A1}.Release|Win32.Build.0 = Release|Win32
+		{4C69A1D2-182A-45D3-AD33-54A8A91B387D}.Debug|Win32.ActiveCfg = Debug|Win32
+		{4C69A1D2-182A-45D3-AD33-54A8A91B387D}.Debug|Win32.Build.0 = Debug|Win32
+		{4C69A1D2-182A-45D3-AD33-54A8A91B387D}.Release|Win32.ActiveCfg = Release|Win32
+		{4C69A1D2-182A-45D3-AD33-54A8A91B387D}.Release|Win32.Build.0 = Release|Win32
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE