Add libfollytestmain.la to mallctl's LDADD

Add libfollytestmain.la to mallctl's LDADD

Summary:
Fixes make check problem for MallctlHelperTest.

Previous make check output:

MallctlHelperTest.o: In function `MallctlHelperTest_valid_read_Test::TestBody()':
MallctlHelperTest.cpp:(.text+0x25): undefined reference to `testing::internal::AlwaysTrue()'
MallctlHelperTest.cpp:(.text+0x2e): undefined reference to `testing::internal::AlwaysTrue()'
MallctlHelperTest.cpp:(.text+0x51): undefined reference to `testing::Message::Message()'
MallctlHelperTest.cpp:(.text+0x72): undefined reference to `testing::internal::AssertHelper::AssertHelper(testing::TestPartResult::Type, char const*, int, char const*)'
MallctlHelperTest.cpp:(.text+0x85): undefined reference to `testing::internal::AssertHelper::operator=(testing::Message const&) const'
MallctlHelperTest.cpp:(.text+0x91): undefined reference to `testing::internal::AssertHelper::~AssertHelper()'
MallctlHelperTest.cpp:(.text+0xe4): undefined reference to `testing::Message::Message()'
MallctlHelperTest.cpp:(.text+0x111): undefined reference to `testing::inte
Closes https://github.com/facebook/folly/pull/448

Reviewed By: yfeldblum

Differential Revision: D3659427

Pulled By: Orvid

fbshipit-source-id: c1445b881bbebdace4defc02024a5184fd821ed6