packages/glibc: Fix mcontext_t error that breaks RISC-V package builds

packages/glibc: Fix mcontext_t error that breaks RISC-V package builds

The riscv-glibc repository version of glibc 2.26 will build for
RISC-V 32bit, but when many packages are built against the resulting
library an 'unknown type name mcontext_t' error is reported. The
definition of mcontext_h in the ucontext.h header file needs to be
moved outside of the '#ifdef __USE_MISC' structure to fix this
issue.

Fixes:
  http://autobuild.buildroot.net/results/5aa9cb29c459f511dc9c4fcf218dc9a842505aa3

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>