boost: make fiber module select the context module

boost: make fiber module select the context module

The boost fiber module uses the boost context module, so it should
select it.

Due to this, the boost fiber module inherits the dependencies of boost
context: it needs exception_ptr handling, and is only available on the
architectures where boost context is supported.

Fixes:

  http://autobuild.buildroot.net/results/56509d315defb95d4ac6e278a9d40cd98f61baa7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Thomas: use only one comment for both gcc bug dependencies.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>