Workaround sdk/shared headers not compiling on macOS

Without this, it tries to build Server SDK even though the project
is marked as ExcludeFromBuild.

Other useful links:
- mirage/ocaml-rpc#45
- https://github.com/premake/premake-core/wiki/kind

Error that this fixes:

```
==== Building Server SDK (debug_x64) ====
Linking Server SDK
ar: no archive members specified
usage:  ar -d [-TLsv] archive file ...
    ar -m [-TLsv] archive file ...
    ar -m [-abiTLsv] position archive file ...
    ar -p [-TLsv] archive [file ...]
    ar -q [-cTLsv] archive file ...
    ar -r [-cuTLsv] archive file ...
    ar -r [-abciuTLsv] position archive file ...
    ar -t [-TLsv] archive [file ...]
    ar -x [-ouTLsv] archive [file ...]
make[1]: *** [bin/x64/Debug/sdk_d.a] Error 1
make: *** [Server SDK] Error 2
```

Server/sdk/premake5.lua

@@ -1,6 +1,11 @@
+-- Workaround for `kind "None"` only being supported for Visual Studio.
+if _ACTION == "gmake" then
+	return
+end
+
 project "Server SDK"
 	language "C++"
-	kind "StaticLib"
+	kind "None"
 	targetname "sdk"
 
 	vpaths {

Shared/premake5.lua

@@ -1,6 +1,11 @@
+-- Workaround for `kind "None"` only being supported for Visual Studio.
+if _ACTION == "gmake" then
+	return
+end
+
 project "Shared"
 	language "C++"
-	kind "StaticLib"
+	kind "None"
 	targetname "shared"
 
 	vpaths {