spandsp3: prevent host include search paths

spandsp3: prevent host include search paths

Note the host include path for libxml2 below. This shouldn't cause an
issue because libxml2 is never used. But better safe than sorry as the
configure script probes other host include paths as well.

/bin/sh ../libtool  --tag=CC   --mode=compile ccache_cc -DHAVE_CONFIG_H -I.  -I.. -I/openwrt/staging_dir/target-mips_24kc_musl/usr/include -I/openwrt/staging_dir/toolchain-mips_24kc_gcc-10.2.0_musl/usr/include -I/openwrt/staging_dir/toolchain-mips_24kc_gcc-10.2.0_musl/include/fortify -I/openwrt/staging_dir/toolchain-mips_24kc_gcc-10.2.0_musl/include  -I/usr/include/libxml2 -DNDEBUG -Wunused-but-set-variable -std=gnu99 -ffast-math -Wall -Wunused-variable -Wwrite-strings -Wstrict-prototypes -Wmissing-prototypes -fvisibility=hidden -DHAVE_VISIBILITY=1  -Os -pipe -mno-branch-likely -mips32r2 -mtune=24kc -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -msoft-float -mips16 -minterlink-mips16 -fmacro-prefix-map=/openwrt/build_dir/target-mips_24kc_musl/spandsp3-2020-08-14-6ec23e5a=spandsp3-2020-08-14-6ec23e5a -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro   -MT dds_float.lo -MD -MP -MF .deps/dds_float.Tpo -c -o dds_float.lo dds_float.c

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>