Resolve ambiguity about SDL headers in fb_sdl

This patch prevents 'fb_sdl' from wrongly using SDL headers located in
the libports repository. We have to make sure to use the headers
installed on the host system.

os/src/drivers/framebuffer/sdl/target.mk

@@ -4,3 +4,12 @@ REQUIRES = linux sdl
 SRC_CC   = fb_sdl.cc input.cc
 LX_LIBS  = sdl
 
+#
+# Explicitly add host headers to the include-search location. Even though this
+# path happens to be added via the 'lx_hybrid' library via the 'HOST_INC_DIR'
+# variable, we want to give /usr/include preference for resolving 'SDL/*'
+# headers. Otherwise, if libSDL is prepared in 'libports', the build system
+# would pull-in the SDL headers from libports.
+#
+INC_DIR += /usr/include
+