Fix compiler errors on MSVC with ENABLE_TOUCH=TRUE

[grorp]

Minetest currently doesn't compile with -DENABLE_TOUCH=TRUE on Windows due to these compiler errors:

C:\mtbuild\minetest-grorp-nonbroken\src\gui\touchscreengui.cpp(43,1): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax [C:\mtbuild\minetest-grorp-nonbroken-built\src\minetest.vcxproj]
C:\mtbuild\minetest-grorp-nonbroken\src\gui\touchscreengui.cpp(49,1): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax [C:\mtbuild\minetest-grorp-nonbroken-built\src\minetest.vcxproj]

This PR fixes these compiler errors.

To do

This PR is a Ready for Review.

How to test

Verify that you can compile Minetest with -DENABLE_TOUCH=TRUE on Windows.

[grorp]

Sadly, the touchscreen GUI doesn't actually work for me on Windows, neither with USE_SDL2=TRUE nor without. It does work on Linux, however.

But at least it compiles now :)