[C++] C project fail to compile on C++ compiler

[voiza]

Due to "virtual" field in aismsg_21 structure, and "true" fields in structures in aismsg_1, 2, 3, 18 and 19 C variant fail to compile.
These words are declared as keywords in C++ and cannot be used as a variable name or structure field.

Changes proposal:

• change fields "true" to "heading" among the project (or to "true_heading", as they declared in Java variant),
• change field "virtual" to "pseudo"

[voiza]

Sent pull request