qwt: fix build when BR2_STATIC_LIBS=y

qwt: fix build when BR2_STATIC_LIBS=y

By default, qwt build a shared library, but when BR2_STATIC_LIBS is set
compilation failed with errors like :
__uClibc_main.c:(.text+0x164): undefined reference to `__fini_array_end'
__uClibc_main.c:(.text+0x168): undefined reference to `__fini_array_start'
__uClibc_main.c:(.text+0x16c): undefined reference to `__fini_array_start'

This patch disable QwtDll to build a static library when BR2_STATIC_LIBS=y.

fix:
http://autobuild.buildroot.net/results/739/739406bb8073d1861933872a47802954d9767634/

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>