doxygen: Set DISABLE_STATIC to be empty

by default its set like this DISABLE_STATIC ?= " --disable-static"

but doxygen configure is not typical autoconf generated so this option is not available, its better to keep it empty so it can work with dustros which disable static libs as policy

---
 recipes-devtools/doxygen/doxygen_1.8.9.1.bb | 2 ++
 1 file changed, 2 insertions(+)

index 27560fdd..eb43e741 100644
@@ -14,6 +14,8 @@ SRC_URI[sha256sum] = "d4ab6e28d4d45d8956cad17470aade3fbe2356e8f64b92167e738c1887
 
 EXTRA_OECONF = "--prefix ${prefix}"
 
+DISABLE_STATIC = ""
+
 do_configure () {
 	./configure ${EXTRA_OECONF}
 

2.19.1