package/google-breakpad: replace references to 'struct ucontext' with 'ucontext_t'

In glibc, since
https://sourceware.org/git/?p=glibc.git;h=251287734e89a52da3db682a8241eb6bccc050c9
the 'struct ucontext' tag has been replaced with 'struct ucontext_t'.
The tag itself is anyway not POSIX - only the 'ucontext_t' typedef is
specified. And that type has existed since at least 1997 in glibc.

Therefore, replace references to 'struct ucontext' with 'ucontext_t',
which works in all versions of glibc, uClibc and musl.

Fixes:
[arm]     http://autobuild.buildroot.net/results/6380341dbb6c114e4452c5cda37da6b44b80d178
[aarch64] http://autobuild.buildroot.net/results/4ecf770df7c984a62082d59f8fab632d3efbe06b
[mipsel]  http://autobuild.buildroot.net/results/e1473a12cf38ccf4dd3ed0f26a8ff9e6b57f0810



Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Arnout: improve commit message]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>