Don't allow 'gender unknown' in voice/language files.

CHANGELOG.md

 ### 1.49.2 - (In Development)
 
 *  Support describing all IPA phonemes in the phoneme tables.
-*  Provide more validation when compiling phoneme tables.
+*  Provide more validation when reading phoneme tables and voice/language files.
 *  Removed support for phoneme equivalence tables. These were disabled in the
    French and German language files.
 *  Allow pre- as well as post-jump rules in dictionary files.

src/libespeak-ng/voices.c

 #include "translate.h"
 
 MNEM_TAB genders[] = {
-	{ "unknown", 0 },
 	{ "male", 1 },
 	{ "female", 2 },
 	{ NULL, 0 }