build: pass win32-dll to LT_INIT

This is the recommended way to support building PE DLLs with modern
mingw toolchains and libtool.

> This option should be used if the package has been ported to build clean
> dlls on win32 platforms.
> If this macro is not used, libtool will assume that the package libraries
> are not dll clean and will build only static libraries on win32 hosts.

See:
https://www.gnu.org/software/libtool/manual/libtool.html#LT_005fINIT
https://www.gnu.org/software/gnulib/manual/html_node/Libtool-and-Windows.html
https://autotools.io/libtool/windows.html

configure.ac

@@ -8,7 +8,7 @@ AH_TOP([#define LIBSECP256K1_CONFIG_H])
 AH_BOTTOM([#endif /*LIBSECP256K1_CONFIG_H*/])
 AM_INIT_AUTOMAKE([foreign subdir-objects])
 
-LT_INIT
+LT_INIT([win32-dll])
 
 # Make the compilation flags quiet unless V=1 is used.
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])