libc: Fix buglet around __progname

508b20ead1710e8c295630d096960e51a65b7e5d disentangled
program_invocation_name and __progname but forgot to make __progname
conditional on __UCLIBC_HAS___PROGNAME__, resulting in getconf
attempting to re-define __progname in case __UCLIBC_HAS___PROGNAME__ was
not defined.

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

include/errno.h

@@ -47,7 +47,9 @@ extern int errno;
 #endif
 
 #ifdef _LIBC
+# ifdef __UCLIBC_HAS___PROGNAME__
 extern const char *__progname, *__progname_full;
+# endif
 #endif
 #if defined __USE_GNU && defined __UCLIBC_HAS_PROGRAM_INVOCATION_NAME__