ffmpeg: install headers on the proper location.

rules/ffmpeg/default.bash

@@ -18,6 +18,7 @@ configure()
     ./configure \
         --sysroot="$cfg_dir_toolchain_sysroot" \
         --prefix="$cfg_dir_rootfs/usr" \
+        --incdir="$cfg_dir_toolchain_sysroot/usr/include" \
         --enable-cross-compile \
         --cross-prefix="$cfg_target_canonical-" \
         --target-os="linux" \
@@ -33,8 +34,14 @@ build()
     $cmd_make
 }
 
+host_install()
+{
+    $cmd_make \
+        install-headers
+}
+
 target_install()
 {
     $cmd_make \
-        install
+        install-libs
 }