package/mono: fixing shared/static handling

Mono should always be compiled with --enable-static, this patch forces
--enable-static also in case of shared only libraries.

Fixes:
http://autobuild.buildroot.net/results/e739d64011fb6a24ccbef136d75944f3bd2eba03/
http://autobuild.buildroot.net/results/7ebe6909da859507639f57da2d4fee46efc062d4/
http://autobuild.buildroot.net/results/03c2243080b026b0c6f4bb64e7dc8dc359f0c8c2/
http://autobuild.buildroot.net/results/406bf830ae84cf9ce4afd5a4ddc265197df42b44/
http://autobuild.buildroot.net/results/894a3e0627244b5e1980c3840629b4627999d252/
http://autobuild.buildroot.net/results/2dad2609834564060dc090919c128b45e289102e/
http://autobuild.buildroot.net/results/46de32cbe68b614763a72303490515f80cfe5bfd/
http://autobuild.buildroot.net/results/b167d2660ca37b896ceb8f1372d88eda6d6397d7/
http://autobuild.buildroot.net/results/17b6aee3d0aaf425412f9f5d89de665de9efedb8/
http://autobuild.buildroot.net/results/af9e5203299819085441fb6ee27e027d58f220b3/
http://autobuild.buildroot.net/results/138e5bd9bd0e2ad6de13278b446d1a48a9b64b17/
http://autobuild.buildroot.net/results/b77d52aaa5face5d5b8afe39f9a32a51b1df500f/



Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>