mnemonics: fix duplicate symbol error

[laanwj]

Fix the following compile error in the tests on gcc 4.8.2:

electrum-words.cpp.o (symbol from plugin): In function `Language::utf8prefix(std::string const&, unsigned long)':
(.text+0x0): multiple definition of `Language::utf8prefix(std::string const&, unsigned long)'
CMakeFiles/unit_tests.dir/mnemonics.cpp.o (symbol from plugin):(.text+0x0): first defined here
collect2: error: ld returned 1 exit status