package/rtmpdump: Fix makefile when GnuTLS is used

In librtmp/Makefile the variable CRYPTO defaults to OpenSSL. When
building with GnuTLS support instead, librtmp.pc includes wrong
libraries:

$ grep Req output/staging/usr/lib/pkgconfig/librtmp.pc
Requires: libssl,libcrypto

using this defconfig

BR2_TOOLCHAIN_BUILDROOT_WCHAR=y
BR2_PACKAGE_GNUTLS=y
BR2_PACKAGE_RTMPDUMP=y

Pass RTMPDUMP_CRYPTO also to INSTALL_*_CMDS to fix this problem.
Patch supposed by Thomas:
http://article.gmane.org/gmane.comp.lib.uclibc.buildroot/129525



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>