yaml-cpp: improving dependencies (resolves #328)

I noticed a build failure for yaml-cpp (#328), that the boost
headers could not be found. Probably since the update to 0.5.2 [1],
yaml-cpp depends on boost. This commit resolves the issue by adding
this dependency.

[1] https://github.com/bmwcarit/meta-ros/commit/da10180fc561ba5da0271476f013d73b6733ea51



Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>

recipes-devtools/yaml-cpp/yaml-cpp_0.5.2.bb

@@ -4,6 +4,8 @@ SECTION = "libs"
 LICENSE = "MIT"
 LIC_FILES_CHKSUM = "file://license.txt;md5=7c6a629da965ebdfba9f6fdb76ab8ab4"
 
+DEPENDS = "boost"
+
 PR = "r0"
 
 S = "${WORKDIR}/yaml-cpp-release-${PV}"