export ewk_ipc_wrt_* API as C linkage

export ewk_ipc_wrt_* API as C linkage

This is necessary due to merged commit http://165.213.202.130:8080/#/c/75191/,
which is to minimize memory size by restricting symbol export as ewk prefixed
ones. Without this patch, wrt package build fails with undefined symbol error.

Bug: http://web.sec.samsung.net/bugzilla/show_bug.cgi?id=11607
Reviewed by: Antonio Gomes, Janusz Majnert, Min-Soo Koo, Piotr Tworek

Change-Id: I590eff2c4b529c3ec46cac427377c6828c52df7d
Signed-off-by: SeungSeop Park <sns.park@samsung.com>