Don't define FMT_LOCALE on OpenBSD

OpenBSD doesn't have strtod_l() (at least under the default
headers), so the class Locale that the macro gates won't compile.

include/fmt/posix.h

@@ -353,7 +353,7 @@ class File {
 long getpagesize();
 
 #if (defined(LC_NUMERIC_MASK) || defined(_MSC_VER)) && \
-    !defined(__ANDROID__) && !defined(__CYGWIN__)
+    !defined(__ANDROID__) && !defined(__CYGWIN__) && !defined(__OpenBSD__)
 # define FMT_LOCALE
 #endif