Add CheckSymbolExists module for FindMbedTLS.cmake

FindMbedTLS.cmake uses `check_symbol_exists` macro which requires
`include(CheckSymbolExists)` before invocation (see
https://cmake.org/cmake/help/latest/module/CheckSymbolExists.html).

Signed-off-by: Azamat H. Hackimov <azamat.hackimov@gmail.com>
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