Solaris: add definition for _POSIX_VDISABLE

(backport <#4103>)
(cherry picked from commit 64dd7f4)

Author: psumbera

libc-test/semver/solarish.txt

@@ -16,6 +16,7 @@ LIO_WAIT
 LIO_WRITE
 PIPE_BUF
 SIGEV_PORT
+_POSIX_VDISABLE
 aio_cancel
 aio_error
 aio_fsync

src/unix/solarish/mod.rs

@@ -1870,6 +1870,8 @@ pub const _PC_FILESIZEBITS: ::c_int = 67;
 pub const _PC_XATTR_ENABLED: ::c_int = 100;
 pub const _PC_XATTR_EXISTS: ::c_int = 101;
 
+pub const _POSIX_VDISABLE: ::cc_t = 0;
+
 pub const _SC_ARG_MAX: ::c_int = 1;
 pub const _SC_CHILD_MAX: ::c_int = 2;
 pub const _SC_CLK_TCK: ::c_int = 3;