astro/wmsolar: Fix build with Clang 6

In file included from wmSolar.c:15:
./wmSolar_mask.xbm:6:12: error: constant expression evaluates to 240 which cannot be narrowed to type 'char' [-Wc++11-narrowing]
0x00,0x00,0xf0,0xff,0xff,0xff,0xff,0xff,0xff,0x0f,0xf0,0xff,0xff,0xff,0xff,

^~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p479076_s338486/logs/errors/wmsolar-1.1.log

In file included from wmSolar.c:18:
./Lunar.h:66:23: error: in-class initializer for static data member of type

    'const double' requires 'constexpr' specifier [-Wstatic-float-init]
static const double SYNODIC_MONTH = 29.530588861;
                    ^

PR: 230880
Approved by: tkuiper@inxsoft.net (maintainer timeout, 2 weeks)