www/firefox: unbreak after r522464

In file included from Unified_cpp_ipc_chromium0.cpp:137:
ipc/chromium/src/base/shared_memory_posix.cc:282:16: error: variable has incomplete type 'cap_rights_t' (aka 'cap_rights')

cap_rights_t rights;
             ^

/usr/include/sys/types.h:247:8: note: forward declaration of 'cap_rights'
struct cap_rights;

^

In file included from Unified_cpp_ipc_chromium0.cpp:137:
ipc/chromium/src/base/shared_memory_posix.cc:283:28: error: use of undeclared identifier 'CAP_MMAP_R'

cap_rights_init(&rights, CAP_MMAP_R);
                         ^

PR: 243213
Pointy hat to: jbeich (landed old version of the patch)