libsigc++: add missing makedep on doxygen

mingw-w64-libsigc++/PKGBUILD

@@ -4,7 +4,7 @@ _realname=libsigc++
 pkgbase=mingw-w64-${_realname}
 pkgname="${MINGW_PACKAGE_PREFIX}-${_realname}"
 pkgver=2.10.4
-pkgrel=2
+pkgrel=3
 pkgdesc="Libsigc++ implements a full callback system for use in widget libraries - V2 (mingw-w64)"
 arch=('any')
 url="https://libsigc.sourceforge.io/"
@@ -12,6 +12,7 @@ license=("LGPL")
 makedepends=("${MINGW_PACKAGE_PREFIX}-gcc"
              "${MINGW_PACKAGE_PREFIX}-meson"
              "${MINGW_PACKAGE_PREFIX}-ninja"
+             "${MINGW_PACKAGE_PREFIX}-doxygen"
              "${MINGW_PACKAGE_PREFIX}-pkg-config")
 depends=("${MINGW_PACKAGE_PREFIX}-gcc-libs")
 options=('staticlibs' 'strip')