Fix symbol visibility on Linux when compiling with -fvisibility=hidde…

[sthagen]

…n (fmtlib#1535)

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

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