boost: fix getchar issue with uClibc and gcc <= 4.8.2

boost: fix getchar issue with uClibc and gcc <= 4.8.2

getchar() is defined as a macro in uClibc. This hits gcc bug 58952 [1] for all
gcc versions <= 4.8.2 and building boost/test fails:

./boost/test/impl/unit_test_main.ipp: In function 'int boost::unit_test::unit_test_main(boost::unit_test::init_unit_test_func, int, char**)':
./boost/test/impl/unit_test_main.ipp:194:18: error: expected unqualified-id before '(' token

To allow building boost/test with uClibc based toolchains with gcc <= 4.8.2 use
parenthesis for std::getchar.

Fixes:
http://autobuild.buildroot.net/results/506/506b3b1b4087318a4247456782ac9e4f128d5db6/
http://autobuild.buildroot.net/results/5f2/5f2a8fb5cb823160542ee721591867b34bd25de6/
http://autobuild.buildroot.net/results/e3e/e3e6002ac5b40f7f03d32ba9da99f6497f01b42f/
.. and more

Upstream status: Pending
https://github.com/boostorg/test/pull/97

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58952

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>