nginx: add patch fixing libxslt library detection

Nginx hardcode the libxml2 include path to /usr/include/libxml2. This
works in a local build as pretty much everyone have the libxml2 library
installed but doesn't on buildbot container as the library doesn't
exist.

This effectively makes the host library leak intro library detection but
doesn't actually link to it as linking is still done with the correct
library in staging dir.

To fix this add a patch to define custom libxml2 include directory
instead of hardcoding it to host library.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

net/nginx/patches/nginx/106-libxslt-fix-detection.patch

@@ -0,0 +1,20 @@
+--- a/auto/lib/libxslt/conf
++++ b/auto/lib/libxslt/conf
+@@ -12,7 +12,7 @@
+                       #include <libxslt/xsltInternals.h>
+                       #include <libxslt/transform.h>
+                       #include <libxslt/xsltutils.h>"
+-    ngx_feature_path="/usr/include/libxml2"
++    ngx_feature_path="${LIBXML2_INC:-/usr/include/libxml2}"
+     ngx_feature_libs="-lxml2 -lxslt"
+     ngx_feature_test="xmlParserCtxtPtr    ctxt = NULL;
+                       xsltStylesheetPtr   sheet = NULL;
+@@ -100,7 +100,7 @@ fi
+     ngx_feature_name=NGX_HAVE_EXSLT
+     ngx_feature_run=no
+     ngx_feature_incs="#include <libexslt/exslt.h>"
+-    ngx_feature_path="/usr/include/libxml2"
++    ngx_feature_path="${LIBXML2_INC:-/usr/include/libxml2}"
+     ngx_feature_libs="-lexslt"
+     ngx_feature_test="exsltRegisterAll();"
+     . auto/feature