boost: fix build failure

boost: fix build failure

Such build failure was reported several times:

gcc.compile.c++
bin.v2/libs/context/build/gcc-i386/release/link-static/threading-multi/fcontext.o
libs/context/src/fcontext.cpp: In function 'void*
boost::ctx::detail::align_stack(void*)':
libs/context/src/fcontext.cpp:25:21: error: 'uintptr_t' was not declared
in this scope
libs/context/src/fcontext.cpp:25:32: error: expected ')' before 'base'
libs/context/src/fcontext.cpp:26:66: error: expected ')' before ';'
token
libs/context/src/fcontext.cpp:26:66: error: expected ')' before ';'
token

Signed-off-by: Florian Fainelli <florian@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/packages@34635 3c298f89-4303-0410-b956-a3cf2f4a3e73