frmts/fit/gstEndian.h defines swap16() already defined by openbsd

[anselm@…]

a trivial bug easy to fix - reproduced under under openbsd3.5 pre-release with 
at head revisions of the gnu tool suite excepting gcc which was at gcc version 
2.95.3 20010125 (prerelease,propolice)

frmts/fit/gstEndian.h defines swap16() swap32() swap64() and under openbsd this 
fails to compile because these are already defined by openbsd.

novice developers may be slightly confused by __extension__ error - but this is 
imply because swap16() is attempting to be macro expanded - so the error 
message from gcc is somewhat confusing.

my solution was to rename these methods and to rename their subsequent use in 
gstEndian.h and in the nearby fitdataset.cpp.

[warmerdam]

Mark,

Could you look into an appropriate patch for this at some point?

Thanks,

[mark@…]

limit scope by wrapping header in namespace