externals/CMakeLists: Add EXCLUDE_FROM_ALL to lz4's add_subdirectory() command

We don't need to build the lz4 CLI tool, or anything else. We just want
to build in the library statically, so we specify this to ensure that.
Now, we don't potentially build unnecessary targets.

externals/CMakeLists.txt

@@ -32,7 +32,7 @@ add_subdirectory(inih)
 
 # lz4
 set(LZ4_BUNDLED_MODE ON)
-add_subdirectory(lz4/contrib/cmake_unofficial)
+add_subdirectory(lz4/contrib/cmake_unofficial EXCLUDE_FROM_ALL)
 target_include_directories(lz4_static INTERFACE ./lz4/lib)
 
 # mbedtls