Compile with VS 2022 and python3

[wenjian-zhou]

Hi, I was compiling mitsuba with scons-python3 branch, my environments are:

• Windows 11
• VS 2022
• Python 3.7

Then I found an error with std::binary_function, the error message says C2143: syntax error : missing ',' before '<'. I found that the error was caused by functional not included in the header files, so I added #include <functional> to mitsuba.h. Then the compilation went successful. Could you check if this is an issue? Thanks!