Fix the compilation errors in gcc-10 with -Werror=ignored-qualifiers option and remove unused lines related to alloca

[s-ymgch228]

I found a build error and unused header file.

1. -Werror=ignored-qualifiers

hidden/cjose/src/jwk.c:1005:1: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
1005 | const cjose_jwk_ec_curve cjose_jwk_EC_get_curve(const cjose_jwk_t *jwk, cjose_err *err)
     | ^~~~~

Currently, cjose_jwk_ec_curve is an enumeration so it seems that const qualifier can not be used.

cjose/include/cjose/jwk.h

Lines 216 to 226 in b8f607d

	typedef enum
	{
	/** NIST P-256 Prime Curve (secp256r1) */
	CJOSE_JWK_EC_P_256 = NID_X9_62_prime256v1,
	/** NIST P-384 Prime Curve (secp384r1) */
	CJOSE_JWK_EC_P_384 = NID_secp384r1,
	/** NIST P-521 Prime Curve (secp521r1) */
	CJOSE_JWK_EC_P_521 = NID_secp521r1,
	/** Invalid Curve */
	CJOSE_JWK_EC_INVALID = -1
	} cjose_jwk_ec_curve;

alloca

alloca() was used in src/concatkdf.c but it is no longer used after #15

[s-ymgch228]

This malloc.h may be unneeded, too. (I don't have _WIN32 host.)