Fix symbol visibility on Linux when compiling with -fvisibility=hidden

Make FMT_API symbols use the default visibility on non-Windows
platforms. Otherwise, one cannot use the generated fmt library when
compiling globally with -fvisibility=hidden.

Fixes compile errors like:

```
../3rdParty/fmt/include/fmt/core.h:757: error: undefined reference to 'fmt::v6::internal::assert_fail(char const*, int, char const*)'
```

Note that the symbol exists, but is local:

```
$ nm -C libfmtd.so.6.1.3  | grep assert_fail
                 U __assert_fail
0000000000233ffa t fmt::v6::internal::assert_fail(char const*, int, char const*)
```

With this patch, the compile error is gone and the symbol is properly
exported:

```
$ nm -a bin/libfmtd.so -C | grep assert_fail
                 U __assert_fail
00000000002366ba T fmt::v6::internal::assert_fail(char const*, int, char const*)
```

Change-Id: I96054e622d9a2ae81907e1b01a1033e629767a91

include/fmt/core.h

@@ -185,7 +185,12 @@
 #  define FMT_CLASS_API
 #endif
 #ifndef FMT_API
-#  define FMT_API
+#  if defined(_WIN32)
+#    define FMT_API
+#  else
+#    define FMT_API __attribute__((visibility("default")))
+#    define FMT_EXTERN_TEMPLATE_API FMT_API
+#  endif
 #endif
 #ifndef FMT_EXTERN_TEMPLATE_API
 #  define FMT_EXTERN_TEMPLATE_API

src/format.cc

@@ -121,7 +121,7 @@ template FMT_API char* internal::sprintf_format(long double,
                                                 internal::buffer<char>&,
                                                 sprintf_specs);
 
-template struct FMT_API internal::basic_data<void>;
+template struct internal::basic_data<void>;
 
 // Workaround a bug in MSVC2013 that prevents instantiation of format_float.
 int (*instantiate_format_float)(double, int, internal::float_specs,