package/gstreamer1: add symlink to gstconfig.h in legacy location

After version 1.4 (the previous one packaged in Buildroot), gstreamer
has changed the location where it installs its gstconfig.h, because it
is architecture-dependent, see upstream commit:
    http://cgit.freedesktop.org/gstreamer/gstreamer/commit/?id=444ceb068424122ceba46d8cc48a53b8b32c3cf5

However, this causes build failures in some pcakges (at least FreeRDP
is known to break) because either they do not use pkg-config (bad!) or,
like FreeRDP, have a broken buildsystem and have not yet fixed their
build failures [0].

In Buildroot, we do not really care about architecture-dependent
locatios, because we only build for one architecture. So, the legacy
location of that header is not problematic to us.

Create a symbolic link to gstconfig.h in its legacy location.

Fixes numerous FreeRDP build failures:
    http://autobuild.buildroot.org/results/438/4384ab572bdcb1d148bcdae5a67d8b0bf26d3049
    http://autobuild.buildroot.org/results/b00/b0060f16c7464c240bc7d3b0ff17c9cb311fe30d
    http://autobuild.buildroot.org/results/cad/cadb2fb90ce66b1fa3051a6fcfb7c8142f420c31
    ...

[0] FreeRDP does have a commit supposedly fixing the build with "newer"
gstreamer versions:
    https://github.com/FreeRDP/FreeRDP/commit/bea27fd919b64ee8d97996409e279e1e83d13594



but it makes the whole build completely fail, as it does not even
detect gstreamer anymore. Sigh... :-(

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>