desktop: Fix whitespace handling around '='
While the spec only allows spaces before and after '=',
xgettext/msgfmt previously accepted all whitespaces including '\n'.
That was causing unwanted concatenation of desktop lines.
Reported by Bastien Nocera in:
https://lists.gnu.org/archive/html/bug-gettext/2019-03/msg00017.html
* gettext-tools/src/read-desktop.c (desktop_lex): Properly handle
space characters before and after '='.
* gettext-tools/tests/msgfmt-desktop-1: Add a test case.
