build: Fix build errors with MSVC.
* gettext-tools/src/lang-table.h (language_table, language_variant_table):
Declare with C linkage.
* gettext-tools/src/read-po.h (input_format_po): Likewise.
* gettext-tools/src/read-properties.h (input_format_properties): Likewise.
* gettext-tools/src/read-stringtable.h (input_format_stringtable): Likewise.
* gettext-tools/src/write-properties.h (output_format_properties): Likewise.
* gettext-tools/src/write-stringtable.h (output_format_stringtable): Likewise.
