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

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Denys Dmytriyenko <denys@ti.com>

recipes-devtools/doxygen/doxygen_1.8.9.1.bb

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