elementary: add poppler to DEPENDS

* poppler is autodetected and then transitive deps are not working as expected (it will remove tiff from sysroot to
  replace it with newer version while building something else against elementary)
  | /OE/shr-core/tmp-eglibc/sysroots/x86_64-linux/usr/libexec/x86_64-oe-linux/gcc/x86_64-oe-linux/4.6.3/ld: warning: libtiff.so.3, needed by /OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/lib/libelementary.so, not found (try using -rpath or -rpath-link)
  | /OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/lib/libpoppler.so: undefined reference to `TIFFSetField'
  | /OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/lib/libpoppler.so: undefined reference to `TIFFClose'
  | /OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/lib/libpoppler.so: undefined reference to `TIFFDefaultStripSize'
  | /OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/lib/libpoppler.so: undefined reference to `TIFFFdOpen'
  | /OE/shr-core/tmp-eglibc/sysroots/qemux86-64/usr/lib/libpoppler.so: undefined reference to `TIFFWriteScanline'

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>