roscpp: add missing header for writev()

[rojkov]

resolves #532

[bulwahn]

I quickly reviewed the pull request and I have no further comments. I will be back from vacation on Monday, then run a test build on my local build machine and merge the pull request.

[bulwahn]

I successfully ran a quick bitbake core-image-ros-world with a clean build directory on this build configuration:

Build Configuration:
BB_VERSION        = "1.35.0"
BUILD_SYS         = "x86_64-linux"
NATIVELSBSTRING   = "ubuntu-14.04"
TARGET_SYS        = "i586-oe-linux"
MACHINE           = "qemux86"
DISTRO            = "nodistro"
DISTRO_VERSION    = "nodistro.0"
TUNE_FEATURES     = "m32 i586"
TARGET_FPU        = ""
meta              = "master:514e9be6b00cd39bb8b2eaf117125109fba17910"
meta-oe           
meta-multimedia   
meta-python       = "master:66b88f7729d24ef2e3469fcaca172f420c4d54be"
meta-ros          = "rojkov-roscpp-532-v1:000032396cf3df674b784df3702bd2be905552c4"