Fix circular include on MS Visual Studio

[ignazp]

Including ucal.h from a C file on MS VisualStudio triggers a circular include, which triggers a build fail.

The circualr include can be resumed as: utypes.h -> umachine.h -> ptypes.h -> uchar.h -> utypes.h

The first reported error is

...\x64\Release\include\unicode\ucpmap.h(12,1): error C2054: expected '(' to follow 'U_CDECL_BEGIN'
70>(compiling source file 'test.c')

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