archivers/libunrar5: fix build on GCC architectures

Add USES=compiler:c++11-lang:
rijndael.hpp:23: error: a function call cannot appear in a constant-expression
rijndael.hpp:23: error: ISO C++ forbids initialization of member 'ctx'
rijndael.hpp:23: error: making 'ctx' static
rijndael.hpp:23: error: invalid in-class initialization of static data member of non-integral type 'EVP_CIPHER_CTX*'

Approved by: mentors (implicit approval)