Unbreak `./configure --with-usrp1` build

Change-Id: I49b385594271ae64a48d4d39ee9fe26d7c95bd30
diff --git a/configure.ac b/configure.ac
index 5cee4a6..7d26ced 100644
--- a/configure.ac
+++ b/configure.ac
@@ -113,7 +113,6 @@
         )]
     )
     AC_DEFINE(USE_UHD, 1, All UHD versions)
-    PKG_CHECK_MODULES(FFTWF, fftw3f)
 ])
 
 AS_IF([test "x$with_singledb" = "xyes"], [
@@ -160,6 +159,7 @@
 AM_CONDITIONAL(ARCH_ARM_A15, [test "x$with_neon_vfpv4" = "xyes"])
 
 PKG_CHECK_MODULES(LIBUSB, libusb-1.0)
+PKG_CHECK_MODULES(FFTWF, fftw3f)
 
 AC_CHECK_HEADER([boost/config.hpp],[],
     [AC_MSG_ERROR([boost/config.hpp not found, install e.g. libboost-dev])])