build-package: API level to 23

* This is for compiling docker-tini, who is asking for sigtimedwait,
  which wasn't exposed in signal.h until API 23.
  termux#899

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>

build-package.sh

@@ -256,7 +256,7 @@ termux_step_setup_variables() {
 	: "${TERMUX_SYSROOT:="${TERMUX_OUTDIR}/sysroot"}"
 	: "${TERMUX_ANDROID_HOME:="/home"}"
 	: "${TERMUX_DEBUG:=""}"
-	: "${TERMUX_PKG_API_LEVEL:="21"}"
+	: "${TERMUX_PKG_API_LEVEL:="23"}"
 	: "${TERMUX_ANDROID_BUILD_TOOLS_VERSION:="27.0.3"}"
 	: "${TERMUX_NDK_VERSION:="16"}"