glsl: simplify the M_PI*f macros, fixes build on OpenBSD

glsl: simplify the M_PI*f macros, fixes build on OpenBSD

The M_PI*f macros used a preprocessor paste to append 'f'
to M_PI defines, which works if the values are only numbers
but breaks on OpenBSD where M_PI definitions have casts
and brackets to meet requirements of a future version of POSIX,

http://austingroupbugs.net/view.php?id=801
http://austingroupbugs.net/view.php?id=828

Simplify the M_PI*f macros by using casts directly in the defines
as suggested by Kenneth Graunke.

Cc: "10.2" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=78665
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>